Welcome to PC krh1973. Sounds like your son has a diaper/baby fetish. I'm surprised he's comfortable doing this around family but guess its better than finding it out by accident. There was thread on this topic, which talks about this extensively. What should you do - I'm not sure. Usually this is a sexual type fetish - this arouses them when they wear diapers and baby clothes.

http://forums.psychcentral.com/sexua...fetishism.html