A.J.
Ok here are my suggestions that have worked for me.
first you could try letting him wear the ONEssies that they sell at gymboree and childrens place for bigger toddlers. they have them all the way up to size 4 I think. That worked with my son for a while because he could not get them off. put some sleep pants on over them.
Second you could try having him wear the full body pajamas and to keem him from un zipping them you could try one of those diaper pins that they are not supposed to be able to undo and stick them selves with.
3rd is have him wear the onesie but have him wear jeans over them and put the jeans on backwards with the button and zipper in the back. (he wont be able to get the jeans off)
I know that one sounds crazy and uncomfortable but that is the one that is for the die hard poop spreaders and it really works if the other suggestions fail.
Good luck with all that...
A. J