For gaming consoles and the ability to play online with friends, XBOX or PlayStation are your main choices.
As mentioned above, first consideration is - which does his best mates have? My son's and their friends have Xbox and so can't play online with others who have PlayStation.
Next is the type of Xbox/PlayStation you get:
Xbox360 (older version) or Xbox One (newer version)
PS3 (older version) or PS4 (newer version)
Also the actual game you buy. For example a new FIFA game is release at the end of September every year. Currently we are on FIFA 20, and FIFA 21 will be released end of Sept.
So, you can't play online across console types (so Xbox online can't play with PlayStation online). But you can play across consoles within the same group (ie someone with XBOX 360 can play with someone on XBOX ONE) as long as you play the same game version (so you both play FIFA 20). But if you have the same console type (xbox or PS) but play different versions of the same game (ie if you buy FIFA 16 and his mates are playing FIFA 19) then they dont be able to play together.
So next question to ask is - which FIFA game are his mates playing? Which Mario game? Minecraft doesn't matter cos there's only one version.
Games are expensive. Circa £50 for a new release. If his mates are playing FIFA 20 then are they (or more likely their Dad's or older siblings) likely to get FIFA 21 when it's out? If so, maybe wait until October before buying FIFA and get FIFA 21.
So, while it's fine (and I would advise) to get an older version of the Xbox or PlayStation console, you need to make sure you get the right version of the game to be able to play with friends online.
My son was playing FIFA at 8 years old, but it is a difficult game and he's likely to lose quite a lot. What's his resilience like? My son persevered because mainly FIFA was his Dad's game, rather than his. Now (aged 10) he's very good. But if your 8yo son will be the only FIFA player in the house, he may well give up if constantly being beat 22-0 or whatever.
Minecraft is a much better option to start off with.
Mario is probably in between in terms of difficulty. It's still not easy and has an element of competition, so if he's likely to cry if he keeps losing games then think carefully.
Whatever game you get, I'd suggest starting off playing offline in order to learn the ropes. Wait until he's reasonably good at the game before going online.