I've been playing through Army of Two co-op with a buddy for a few weeks now and it's pretty fun. Not great (the aiming is clunky), but definitely fun.
Crackdown is one of the best co-op games I've ever played. Check out this video I made of some of the stuff that happened while me and a buddy were playing co-op: http://www.vimeo.com/868318
(it really picks up in the middle of the video when we ditch the cars, but check out the whole thing).
Also, there are some pretty good ones on the XBLA right now. Aces of the Galaxy is similar to Star Fox type gameplay and has online co-op. Schizoid and Aegis Wing are both a blast to runthrough in co-op online as well. If you can play with him locally, the same goes for Geometry Wars 2 (no online co-op in that one though).
There are a BUNCH of good looking co-op games around the corner too. Too Human looks like a lot of fun in co-op (comes out this week I believe) although the campaign is kind of gimped in co-op I've heard (no cutscenes or story elements) and Mercs 2 comes out at the end of the month and the co-op in that looks AWESOME.
Then, beyond that and before the end of the year lots of good looking co-op shooters. Gears 2, CoD: World at War, and Left 4 Dead all look like a blast.
Seriously though, me and my buddy have had Crackdown for a year now and we still go back and play the co-op occasionally. Once you level up your characters that game is incredible.