The question regarding disability relates directly to your ability to acquire a disability tax certificate with CRA. It is a substantial credit that can be transferred to you if your son does not require any or all of it for his return. The idea behind it is to relieve the person who is disabled or the care giver of some of the tax to allow more money to be spent supporting the disabled person. It is a very good deduction and well worth looking into.
If your doctor refuses to sign anything, you need to find another doctor who is more accommodating. Signing needed medical forms is part of his job and he perhaps needs a reminder. There may be a specialist who has seen your son that will provide the needed signature.
Another great benefit that the disability tax certificate provides is the ability to provide the documentation to satisfactorily prove that the person is disabled. He or she is then eligible for the Registered Disability Savings Plan. The federal government contributes a substantial portion of the deposits to this account, which will provide enough money to care for his needs when you may not be there to do so.
To refuse to look into this further would be doing yourself and your son a disservice, but if you prefer to not deal with this issue, you will have to answer no to the disability.
http://www.cra-arc.gc.ca/E/pbg/tf/t2201/t2201-fill-12e.pdf
