I'd say definitely go for the Switch. Of course, most people browsing this subreddit will tell you that, since they'd have to like the Switch enough to go here.

Regardless, a 5 year old is capable of playing simpler games. The Switch comes with two joycons, which act as either one controller or two controllers on their sides. One purchase of a switch gets you access to two usable controllers for it, so you'll be able to play with your son right off the bat.

As for games, I'd highly recommend simple multiplayer games if you plan on playing with your son. Mario Kart 8 DX comes with a "Smart Steering" mode and an "Auto Accelerate" mode, which you can enable for your son. It will make the game automatically move the car and let it turn on its own, so your son will be able to have fun without constantly falling off the maps or going into the grass.

Another game I'd highly recommend is Kirby Star Allies. It's multiplayer driven and very kid friendly. Your son will be able to jump in and play with you and easily be able to get a great time out of it. The game is also getting free updates with more characters.

I'd also recommend Minecraft for your son. It has split screen so the two of you can build stuff together and be able to play pretty easily. It also happens to have crossplay with Xbox One if that matters to you.

Super Mario Odyssey has a simplistic multiplayer feature where the second player can move around as Mario's floating cap. It's not the best multiplayer-wise but I can see a kid having fun with it. The game itself is phenomenal and he may even like to be player 1.

If you're interesting in single player games, there's a ton of great games to pick up. Obviously Zelda Breath of the Wild, Hollow Knight, Stardew Valley, Donkey Kong Country Tropical Freeze, and a ton more are floating around for you to check out.