It'd be basically you giving him an “I owe you X bitcoins” piece of paper that he couldn't do anything other than wait until he's 18, to redeem it.
This reminds me of
Using Locktime for inheritance planning, backups or gifts: Coingifter could create a time locked transaction that's only valid when the kid turns 18. This way, he can receive all the information he needs to redeem the coins already, but can't use it yet.
Pros: Coingifter can forget about it.
Cons: The kid still has to trust Coingifter doesn't take the money earlier. And it requires more understanding of what you're doing because it's more complicate to create/redeem.
Do I have any other option, him being a minor? So how do I give him Bitcoin in a way that ensures he cannot tinker with it?
How about this: Give him half the Bitcoins now, and tell him you'll double it if he doesn't move it before he turns 18!
I am concerned that he might try to buy more using my debit card credentials. Not saying he would, but just want to make sure.
Bitcoin has nothing to do with your debit card. Owning Bitcoins doesn't give anyone access to whatever other payment methods you used to buy them. If you're concerned about this, it sounds like you should learn a bit more about Bitcoin by yourself. That's good for 2 reasons: first, you may benefit from it in the long term, and second, you can teach your nephew how it works.
If I am going to sell the idea to his parents, I know they will want reassurances that there is no danger of him getting himself or anyone else into financial trouble.
Unlike credit cards, you can't spend Bitcoins you don't have. That should reassure them he can't go negative.
