I would show the 12 year old some pictures of other kids that are into wearing diapers.
Explain to the 12 year old how comfortable diapers are and that they feel great when wet or poopy or both.
Explain that they can pee or poop when they need too and they don't have to get up to use the toilet.
Or if you would prefer you could put the 12 year old in a diaper overnight while they are sleeping.
Then they may come to enjoy it.
I'll try to think of some more ideas and post them.