Aye, as @Gavinator said, it’s fairly good value as long as you are going to be using it to its full potential. I managed to snag 4 months of Ultimate for £1 as I had Xbox Gold until December so it combined the two of them into the Ultimate pass.
Keep in mind that it also comes with access to the Game Pass for PC so if you register it for your son under his or your account, you can log into the Xbox app on Windows 10 and get tons of games on that too for the £11.99.
I doubt they’ll ever discontinue the regular Xbox Live subscription as there’s tons of people still using the Xbox 360 which can’t get the benefits of Game Pass, only Xbox One has those benefits. You can still use the Xbox Live Gold on Xbox 360 however, that was something I was a tad confused on as I still play on the 360 often for the classics.