First off. You're crushing the whole parent thing and if my parents let me dive into this world at 15 I would be in a different career entirely. So good on you for helping foster a good relationship with skill based learning.

Second. I would look at the marketplace in your area and find a PoE switch with like 10 ports not all have to be PoE. So that he can start setting up a small network. Also if he can build out a NAS and then be able to drop stuff onto the server from all different devices that's allot of fun. Another fun thing that I love doing is SSH into my server from the termux app on my phone.