I'm not familiar with sleep sacks but had the same thought as homeschoolmom, could you put it on backwards and zip it up or does it not have a zipper?

I also thought the same as Rachael and think he will likely undo the pin or pop it open and hurt himself.

I'm thinking consistency is going to be the answer here. Just keep going in and putting him in the playpen or back on the mat and hopefully he will eventually "get" the rule that he has to stay put until it's time to wake up