Add sim to my contact for my son

My son is getting his first phone soon, on his birthday in October. I want to get a Sim and contract that is connected to my contract, and was hoping there is a good deal out there. I heard about partner contracts but am not exactly sure how they work.

I don't want to give him much data, as it will be primarily for telephone contact and used in WiFi. 1gb is more then enough. I want to keep the cost was as low as possible. If it helps save money, I am also eligible to extend my contract, and will happily do that. I will probably give him an old phone I have already, but if there's a cheap Android device that makes sense I will consider it. I am not interested in mid or high end phones.

Thanks for any advice!