My son goes to a special need place where they teach him stuff like 1+1. easy stuff like that. he's 13 & he still can't learn this. what should i do?

Plan ahead: A mentally challenged person needs to acquire certain skill sets to function in the least restrictive environment as they grow older. Learning math is not as important as learning to dress, bathe, toilet, and be aware of their surroundings. If he can acquire the basics, he can live in a sheltered environment when he is older, with others helping with basic needs. Be supportive, but realistic.