Here are my thoughts, including games that are already recommended. I’d go for any of these:
If you’re interested in co-op, Mario is a good shout. The new Super Mario 3D World + Bowser’s Fury has online multiplayer and I’ve been playing it with my girlfriend (I think New Super Mario Bros U Deluxe only has local?)
Overcooked 2 has online multiplayer, the first one is local-only unless there’s been an update. The second one is a bit more polished and well balanced so the better choice anyway.
Splatoon 2 is a good shout as well for the reasons above. It has a competitive and a co-op online mode apparently. Alternatively: Plants vs Zombies: Battle for Neighborville comes out in 4 days. Splatoon is probably the safer bet.
Phogs is a game where you each control one head of a double headed dog in a charming colourful environment. It’s new and silly and I enjoyed a demo I played a while ago.
Fall Guys is a really good recommendation but doesn’t arrive until later this year and involves lots of losing (but in an environment against 60 people maybe is a good way to prove that losing doesn’t always oppose fun) It’s colourful and funny and you can dress your little guy up to show off how many wins you’ve got which, if Fortnite is anything to go by, kids love.
Marvel Ultimate Alliance 3 has online co-op and hits a sweet spot where it’s a recognisable IP and simple for kids to get into. It’s also probably incredibly tedious if you’re not interested in Marvel. Same goes for Minecraft Dungeons where it’s pretty fun but probably not for you if you’re already put off by the original game.
Human Fall Flat is maybe a bit too janky and complex to recommend fully unless you’re interested in exploring that space. Out of all the games it actually has the best player-interactions where you’re able to touch, bump, grab each other and feel like you’re interacting in a way that’s super engaging. It’s a simple physics based puzzle game that I found really enjoyable in multiplayer.
Stardew Valley is probably also a good idea, if he enjoyed Animal Crossing. I haven’t played the multiplayer but seems similar in that you’re both inhabiting a small town together, with enough new bits to separate it.
Presented in roughly the order I’d go for them in, but hopefully with enough info in there to help you figure out which ones are appropriate/interest you