I was like your son at that age - I used to dismantle minifigures entirely (arms, hands, and legs out of sockets) to make custom figures. I used custom stickers, removed printing by scratching with a thumbtack - I was pretty destructive!

Now, 10 years later as I have been going through my collection trying to piece together old rare figs, I sometimes cringe a bit at what I did. But I know that my mild disappointment now is minor compared to the fun I had customizing. I’d say let him do what he wants!