Agreeing with Viral_Frog here, Killing Floor 2 is a great coop game. When I first saw it in the Steam store (well, Killing Floor 1), it sounded like my least liked kind of coop , wave defence. And it is that, but it is good. Then Killing Floor 2 came out and I switched to that and I find it better.
The guns feels great, there is classes that excel at different weapons, but the skills are passive, firing guns is the main course and any class can buy any weapon, they just wont be as good with them. There is very good feedback in the game if you hit or not, the zeds lose limbs and get decapitated, which could of course be a problem, depending on the age of your son.
All in all, if you’re looking for a good coop game of the “play a map for 20-60 min with a set amount of waves that culminates in a boss fight” variety, Killing Floor 2 is worth looking into, at least giving it a chance if it ever have a free weekend.