I guess it really depends on his attitude towards games. Kids can be little shits who don’t understand that playing the game is the fun and not necessarily winning, then they throw massive fits when they lose. I suppose that’s the best thing to keep In mind is that and try to reinforce it.
I have 3 nephews that all love Smash. I think they’re aged like 8, 11, and 13. I kept insisting that we do timed mode so they all could keep playing the whole time, but they all wanted to do stocks; yet they were still having a blast and didn’t mind losing first, so I guess they’re cool with it. I just try to take it easy and not react so fast to everything, generally trying to pull off fun stupid shit instead of the logical safe choice (Warlock Punches, etc)
Just play super casually on your end, with characters you’re least familiar with, and gauge his experience to see what he finds most fun. If he’s super receptive in general, and isn’t just doing it for casual entertainment, you can always teach him what he should be doing.