Get some footie pajamas that zip up and cut out the feet so you can put them on him backwards. Another mom suggested cutting out the neck to avoid "chapping" or irritation at the throat (since the back of the pajamas go higher than the front does) but I found that to be unnecessary. It's one of those things that LOOKS like it should bother them, but it really doesn't. It's not like the fabric rubs around a lot while they're sleeping.
Another option is to put them on him front ways, but with a t-shirt over the top to keep him from messing with the top of the zipper/clasp. This might work for a while, at least. It's a little hot, though, so you'll need to keep that in mind.
I've had other moms tell me to put duct tape around the top of the diaper at night to prevent "tampering" but never actually tried it since I didn't want all the hastle when I changed the diaper later. But looking back, I went through a lot of hastle anyway so perhaps I should have given that a shot. :-)
Good luck.