Have a girlfriend come over an "babysit him" while he's in diapers and dressed like a baby.
Make sure he's wearing a cute baby outfit and the locking panties over his diapers.
Explain to her clearly wear the extra diapers are and what she is to feed him and when he is to go to bed (early). Tell her he can watch some cartoons with her if he is being good.
Show her how to tie him down so that he can't move if she needs to change him or if he's being a brat.