I have a stepson that recently turned 8. If you're looking for platformer games (or sidescrollers), there are a few that he may/may not like, depending on his attention span, interest, etc...
Tommy Tronic is an easy game, much like the old platformers... kinda reminds me of Commander Keen. It's fairly pretty, but simplistic in application. It was fun, and I got it on sale for $5 or less during a sale on steam. It allows the player to have a lot of hit points, so it's not discouraging to really young players. It's also slower paced so it has less of the "twitch gaming" like super meat boy does.
Here's my number 1 recommendation:
Rayman Origins. This game is a sidescroller, beautiful art, and tons of fun for the whole family. It has challenges (like time trials, get X number of points, etc) for the older players, as well as allowing normal gameplay for kids playing through.
The art is fantastic, cartoony, and fast paced. It keeps the attention of our 8 year old for an hour or two at a time (which he'd rather go outside and skateboard or ride bike than play most games). The story is minimalist in concept, as most platformers are.
It has various different styles of stages, which challenge you in different ways. Some levels become flying shooters (on the back of a spitting misquito), others are timing sensitive so you have to run fast, typical platformer levels, and Boss levels. They mix it up so it seems refreshing in the 60+ levels in the game.
And here's why I recommend it the most for you... Multiplayer! It allows you to jump in/jump out of a game at any time. You can hop in and help your kid with a level he's having trouble with, then hop out if you want. It also makes some areas easier, and you have "infinite lives". When you "Die" and someone else is on the screen, you simply turn into a bubble. Steer your bubble over to the other character and they "pop" you, and you're back into play. It's that simple. And if everyone dies, the stage is designed so you just start a little way back, rather than needing to start the whole level over.