Yes. 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 his 18, to redeem it. If you consider this a nice way of delivering him his gift, then do it. I wish my uncle/aunt had bought me bitcoins as a gift six years ago.

However, I'd appreciate it even more if they had taught me how it works and what's the purpose of it.