You should read the decision in the Hilewitz v Canada (MInister of Citizenship and Immigration) case, the link is here:
http://www.immigration.ca/permres-gii-medical.asp
Basically the government said 'no' to the intellectually disabled child, but on appeal the case was overturned as the parents were able to show that the officers hadn't taken into consideration the individual circumstances of the case and had used a 'cookie cutter' approach to the case. As such they are now required to look at these sorts of medical issue things much more closely.
If I was you I'd get letters from your specialist, therapists (OT and PT) and GP and (providing they say the kid won't cost the government any outrageous expenses) send them in with the application form. Add in a covering letter from yourself explaining his needs, his potential future needs and how you and the family will deal with them. If necessary get a lawyer or legal eagle to write this for you.
Hope that helps.
Graeme