Hopefully, two of your criteria for choosing your assistance dog were that he would be physically and mentally capable of doing any task that you would require of him. As the trainer as well as owner/handler of a service dog, you will also need to consider WHEN he will be physically and mentally ready for some of the tasks and what impact they may have on him.

Physical Needs of Service Dogs
A dog that is asked to train for and carry out tasks that he is not yet physically mature enough to do may negatively impact his overall health. Any behavior that is repetitive especially on a daily basis, that is done on a hard surface, involves jumping, or puts stress on joints (such as rearing up) should be closely evaluated for when they should be trained. Hips, elbows, knees and spine are the most affected.

A dog’s bones are not fully formed until they are 12 to 18 months old. Smaller dogs develop sooner than larger dogs. By that age, the bone plates have closed and if they have formed properly, your dog is more likely to be sound. If there is significant or ongoing stress as the plates are forming, the bones may be malformed and the damage can become permanent.

Overall physical stamina is another consideration. A 6 month old puppy has less stamina than a 2 year old dog and a senior dog also likely has less than the 2 year old. You need to choose the number and type of tasks to train accordingly.

Since bracing work is especially structurally stressful, make sure dog is structurally sound and suitable build for this work and wait until his bone plates have closed before starting the training. Have a vet assess your dog. Pay particular attention to the technique you use to teach a brace as you want to make sure you are distributing your weight over the shoulder area evenly, not putting any stress on the spine and placing your weight so the stress goes straight down through the dog’s legs toward the ground. Think of a cane being held upright versus being held on angle. The one on the angle put the stress on the cane and it may snap. The cane held upright puts the stress on the bottom tip on the ground where it should be.

Pulling a wheelchair is another physically stressful task as is opening a heavy public door. Start with a harness that is designed for the task, and has been properly fitted. Make sure your dog is physically mature and that you slowly condition him to do the amount of pulling you need on a daily basis. Think of your dog as an athlete: every aspect of their physical training (weight, distance, speed, duration, etc) should be increased in small increments and trained one aspect at a time. Then you can bring the aspects together by adding two together, then three etc.

Mental & Emotional Needs of Service Dogs
Mental and emotional levels and requirements also need to be considered when considering training new tasks. Does your dog have the self control, body awareness and mental maturity to complete both the training and implementation of each specific task?

Some dogs won’t be ready for certain tasks until they are older. For example, your dog might be too mouthy (unaware of the impact his teeth have on you) to carefully pull off a sock without injury to you or too exuberant to paw a light switch without scratching the wall. He may not yet be physically aware of his body to safely navigate close to you in a wheelchair. You can choose to start training these but not expect proficiency until he is older, or you may choose to wait to train them.

Mental stamina increases from puppyhood to maturity and beyond. Start with simple tasks, and train more complicated ones. Start with a few and build to many, alternate training tasks so you don’t overload him in training.

Consider how long your dog can focus on a task or tasks without getting fatigued - a very common cause of refusal. Stop well before you get to that point. Better to leave him eager for more than getting tired of what you are training.

Your dog needs to feel connected to you. Regular training and play builds that bond (remembering than training should always be fun!) Of course, your dog needs daily love and attention from you (and maybe others).

Overall Maintenance of Health
Maintaining your dog in good working condition is critical to his performance. Ensure he is getting the exercise levels he needs for a dog his body structure, breed and age and getting quality food.

Daily exercise builds muscle tone, helps with body awareness, expends extra energy and stress and helps keep him at an ideal weight. It also helps to keep a more active dog calm.

Factor in how much exercise he gets doing tasks for you, then make up the balance of his needs with other forms of exercise. You may need to be creative with how you exercise him if you handle him alone and have physical or balance disabilities. Teaching him to chase a piece of fur dran along th ground on a string, or the tip of a long target stick, retrieve a ball, pacing alongside your wheelchair, sending him to run around objects at a distance or targeting a spot on the fence etc may be options. Directed exercise may also help him to bond with you since you are the provider of this resource.

An overweight or obese dog is less likely to want to work for the handler, has lower energy levels, may be sluggish in performing them and the extra weight puts him at risk for heart and joint-related health problems. This makes it difficult to maintain his performance and training.

Tip:
To figure out if your dog is overweight, gently touch the tips of your fingers to his ribs. You should be able to easily feel the rib bones under his skin. If there is a layer of fat, he needs to lose some weight. A quick visual check (not as effective) is to look from above to see an hourglass shape with his chest and hips being wider than his stomach area. If in doubt, ask your veterinarian.

Your dog needs quality food suited to his age and energy output.

For large-boned dogs, it is important to pay attention to how much calcium is required for proper growth during puppyhood. Consult your vet, but a heads up that too much calcium can be bad for large breeds or big-boned dogs as it promotes the bones to grow too quickly and become less dense, and therefore less able to withstand stress. If your breed is prone to hip or elbow dysplasia, an Orthopedic Foundation for Animals (OFA) screening test at 2 years of age done by a veterinarian may be needed.

Since you are most likely using treats to train, you can fend off obesity by removing the equivalent of food from his daily ration that you feed him in treats. For a handful of treats, remove what you think the caloric equivalent is for his food. You may also be able to use his daily food as the reward for training, providing it motivates him enough to work. In more distracting environments, or to motivate him to do new tasks, you may need to use higher value, but healthy treats, such as small cubes of liver, chicken, cheese etc.

An easy way to reduce caloric intake is to remove one fifth of his regular meal and substitute it with cooked pumpkin or other squashes, boiled frozen green beans or other vegetable that he will willingly eat. Once he gets to his ideal weight, you may need to experiment with how much food he needs to keep him at a stable weight.

On a daily basis, every dog needs a balance of rest and recreation time suited to his specific needs. After performing longer than usual or in stressful situations, it is important to give your dog sufficient rest and recovery time afterwards. Giving him a day off, or periods of time where he can remove himself from the stress is needed to keep him happy and healthy.

When in the midst of performing, a short break from the situation, a change of task or creating an opportunity to physically release stress by chasing a ball or playing tug may help him deal with the stress in a appropriate way. Then he can get back to work.

A Study of the Impact on Service Dogs for Autistic Children might be of interest.
Factors Affecting Behavior and Welfare of Service Dogs for Children With
Autism Spectrum Disorder