Yes! Icrpg will be perfect for you guys!
My advice here will differ a bit from Alex’s very solid advice. Not that it’s better, just a different approach the I used with my 6 year old getting him to play with my two older kids (10 and 12 yr. Olds).
I went with three stats: Strength for anything melee combat related and other things that make sense to require strength. Athletics for ranged combat, dodging, jumping, etc., basically Dex. And Smarts for all magic, knowledge, wisdom, Intelligence, etc.
I would absolutely recommend running the ICRPG adventure “Orvald’s Tower”. It is a great intro adventure to ICRPG as it really shows a lot of the really cool stuff from the system in a pretty clear way.
Now, for duet style play, I would load your son’s character up with a powerful weapon early as a pick up. Kids love “finding” powerful weapons even more than starting with them. Give him a couple of healing potions or make sure he finds them early. When I ran my kids through this one, I dropped a healing fairy (Legend of Zelda style) just past the spiral stairs. Basically provide opportunities to heal up. When my kids characters “died”, they were just respawned back at the beginning of that floor videogame style. One of the cooler parts of this adventure for teaching the game is that each floor has a central obstacle that is more or less static making it conducive to “try again”.
Above all else, the best advice I can offer is this: make it fun for both of you! Cheer him on, encourage, him to think outside the box. Show him you’re on his team even though you’re running the monsters or rolling the maelstrom.