We just moved my daughter about 6 weeks ago and we thought we had it made because for the first month she never even tried to get out of bed. Well, after a month, she figured it out. We did a couple of things. First, we moved her night table closer to her bed and left a few books on there that she could look at. There is also a light on her night table that she can turn on just by touching the base. We read stories to her right before bedtime, so we told her that she had to wait until she woke up and then she could turn her light on and read (this includes naptime). She also sleeps with a blankie and some dolls, so we told her if she got out of her bed again, we were taking them all away. If there is anything that he sleeps with that is really important, you could threaten to take it (you have to follow through though and not give it back if he is going to take you seriously).
We never had to actually take anything. She knew I meant business and hasn't gotten up again since. I think the best thing to do is let him know that he can get up to play AFTER he wakes up from his sleep and/or tell him that he can't have his special things in bed with him if he doesn't stay in it. This has also worked for my friend's son (that's who I got the idea from). Best of luck to you!