To purchase a gold subscription individually you need to do it on your son's account. If you don't have a PPC (Prepaid Code) you can go over on the dashboard to Settings on the right, then to "Privacy & Online Settings" and enable purchases. It will ask
you for your password. Then add you CC in Account Management (also in Settings) then in My Memberships you can upgrade to a gold subscription. After you purchase the subscription go back to the Privacy & Online Settings and remove the ability to make purchases.
Another option is Microsoft Store online or amazon.com you can buy PPC's from them, then redeem the code onto your son's account, and no worries about the CC on the account. ALSO on amazon.com they have deals that are sometimes cheaper, like a year is $59.99
plus tax (depending on your area) but on amazon they're around $50, and I've seen it for like $40 last week. 3 months is like $21 instead of $25. Just make sure when you use this option you are purchasing "Online Game Code" it'll say it right next to the offer.
I hope this helps!