You can do Coop on a single system but it has to be in offline mode where you don't obtain XP like you do in the online coop mode. Still fun for playing together though and you will still get the achievements.
Just have him sign in as a guest and you can play online. It will even save his progress if he levels up. You just have to sign out and back in for it to "catch up". It's an odd little hiccup but at least he's not stuck with the default classes.
100 Stories, of 100 Tons of 110% Ten-Speed Terror. Straight Out the Grease Yo, Like...Bam.