Just because he isn't talking doesn't mean he doesn't understand every word you say. So continue to verbally praise him. If you wanted to reward and reinforce good behaviour you could have a good behaviour card (not sure what symbol to use), and you could give it to him when he has been good. He could then trade in that card for maybe a sticker and he could have a chart where he has a list of things he would like eg. going to the cinema, or going swimming etc and when he has an agreed number of stickers then he gets the reward. Only you have an idea of how easy/difficult you can make this system. It may be that he wouldn't understand it, or you may need to start with him only needing 1 sticker to get a reward, and you would need to repeatedly talk him through the process ie. you get this good behaviour card because ......., you can swop the good behaviour card for a sticker, with your sticker you will get a reward/treat that you have chosen.
It was a pleasure to read both of the above posts You both sounded like very interested caring Moms, I have also heard of the sticker reward system and the friends gave it a big plus .
I like your post. I afree with the comment about being nonverbal do0esn't mean your child doesn't understand. Actually if you want some fun, try searching around the internet. There are a few nonverbal autistic people that blog. When you read their blogs, they sound no different than anyone else.
Here's something interesting:
http://autism.about.com/b/2008/02/24/your-opinion-requested-are-non-verbal-people-with-autism-intellectually-capable.htm
Have you tried looking into helping your nonverbal autistic child use a computer? Maybe it would help, maybe not. It'd be worth a try.