I have also considered putting it in my own name and transferring it to him legally when he hits 18. Is there any way of giving him "read-only" login rights to an account so he can check the value without being able to tinker with anything?
It looks like you're want to use exchanges/custodial wallet isn't? Well it's very not recommended to leave your exchanges for a long time, we don't know the future what the future of your exchanges. Only in 1-3 years there's a lot change on each exchanges, it could be hacked, exit scam, shut down, popular etc.
Yeah you can create
watch only wallet only by importing the public key
(not private key or seed) of your wallet, so your nephew can't do anything except read the balance and transaction. It's only for non-custodial wallet.