You need Gold for online, not for solo play.
If paying through Xbox console you need a bank card / Paypal. You get a free trial for a new user, can't remember how long, I think sometimes they offer a month, some games come with 14 trial codes (hard copy of games, not figital)
It's £6 per month (sometimes an offer of £1 for a month) £40 per year (obviously cheaper)
If you go to the likes of CDKeys it is cheaper. the price flutuates between £33 and £37 (not as cheap as it used to be for various reasons) if you go yo CDKeys, thr offer 5% discount also if you like their facebook page.
If you take subscribe to Gold you get 4 games a month (Games with Gold) 2 Xbox one and 2 Xbox 360 games.
These games are yours to keep as long as you subscribe. If your subscription lapses , these games will be unplayable until you re-subscribe. So it is worthwhile addingvthem to your Gold account every month (free is free)
Your son will need to friend request / accept friend request from his friends to join games with them.
Sorry for the wall if txt, also for reposting the things others have already mentioned (props to them)
Just thought I'd put everything.