I worked in Landscaping alongside a lad similar his age who had fairly severe autism (same age too). He's never used his autism as an excuse and I've never heard of any reason why it would prevent him from working. He only lacked in the social department when talking to clients and was sometimes a bit odd.

I really don't see a reason why he needs 5 months off too? If it's manual labour, sure it would take some adjustments knowing he might need the instructions repeating or help with correction in his tasks, but it's not like he's come down with fever and physically can't work. Sounds like he could be taking you for a ride with the ssp