He would have no right to trade or amend things in any way as I retain the private key (?)
When he is 18 I could fully transfer everything to him legally.
Why not wait until he's 18? If you give him a watch-only wallet now it's basically an "I Owe You" that's completely useless. But worse: many people have been scammed when they thought they bought Bitcoin, but all they got was a watch-only address. He shouldn't think having an address is the same as owning Bitcoin.