Me and the boy having been playing Mario Odyssey. I'm going to say it is one of the best games I have ever played and he loves it. It somehow has enough to make some part really challenging but still doable by a 5 year old.
Anything that competes with Odyssey I should be getting for us to play, doesn't have to be strictly 2 player as we take it in turns if single player?
Switch availability only other wise we'd get it on the xBox.
I absolutely loved Mario Odyssey too.
Have you got Mario 3D all-stars? Not sure how much that is to buy on the digital store (don't even entertain getting the physical version as it is like hens teeth and fetches stupid money)
That 3D allstars game has Mario 64, which is still a great game but maybe the graphics will be a bit of a let down for your son.
But Mario sunshine is a great game (from the gamecube)
And it also has Mario Galaxy on it which is really good and is what came before Odyssey so not a million miles away with the concept.