A 12 year old boy really should be using the toilet but if they're wetting the bed and want to wear a diaper for bed wetting then yes you should let them.
Also if the 12 year old boy wants to wear diaper just because they want to wear a diaper then I would say you should let them then as well.
No harm is gonna be done by letting your 12 year old boy wear a diaper because he wants too but if he uses the diapers then make sure he cleans up properly to avoid a rash.