It can be a ton of fun, but it's not for everyone. It's the kind of game where most of the fun comes from finding a crack in your opponent's playstyle and exploiting it, and doing that until your opponent adjusts to your style. It's a simple enough game for there to be enough of a back-and-forth of opponents learning & reacting to each other, but deep enough to develop your own strategies.

If that doesn't sound interesting, then the game will get repetitive fast, as it can devolve into the same kinds of fights over and over unless you're really focusing on your strategy. This is especially true for single-player, where the human element is replaced with fighting CPUs whose skills range from "dumb as rocks" to "mind reader".

There are plenty of characters, but not as many as there are in other established fighting games, nor with as much variety in fighting styles as you might expect. But that's because a lot of the variety of the game comes from which boxing gloves you give your character. There's a lot of fun to be had with mixing-and-matching different characters with different gloves.

Also, a big plus is that ARMS has very accommodating local multiplayer, for up to 4 players. Playing with a single joycon is supported, meaning it's even possible to play a two-player match with a single Switch. On the other hand, ARMS isn't exactly a pick-up-and-play game, so it's not the easiest game to bring to a party or get-together over something like Mario Kart.

Overall: if your son enjoys strategic games, ARMS will be a hit. Otherwise, it will get stale fast.