This may sound counterintuitive, but something to try may be to lay off praising so much the "big boy" stuff for awhile and let him be a "baby". Let him wear diapers/pullups, offer to feed him, etc. BUT, then he also has to be on the baby's routine. So when the baby goes down for a nap, he has to go to sleep too because "remember, you are a baby too, right? and babies need a lot of sleep." Also, babies can only do "baby" things, not cool stuff like a "big kid", so if he wants to be a baby, fine, but then he can't do all the fun stuff he usually does too. I am guessing he may change his mind after a little while of that and remembering all the cool things and privileges that come with being older.