It's very common for a 12 year old boy and for other kids of any age and even teens and adults to like to wear and use diapers.
There's a whole community of adults who like to wear diapers and have liked to wear and use diapers since they were kids.
Wearing diapers is not gonna hurt your 12 year old so I wouldn't worry about it and as long as your son cleans up properly after wearing and using the diapers and he buys the diapers himself with his allowance or money he has then it's okay if your 12 year old son or even older wants to wear diapers.
There's worse things your 12 year old could be into and diapers is a pretty tame thing for him to be wanting to wear.
He wore diapers as a baby so there's nothing wrong with him wanting to wear diapers as a 12 year old or beyond.
Your 12 year old is likely just a diaper lover like many other people are as well.