What is a good 2 player game I can play with my son (we have 2 pc's). RPGish. Thanks

MMO -> Massively Multiplayer, so lots of people. Of course it's still possible to team up with your son, there'll just be more people around too.
And, what do you prefer (other than RPG)? SF, Fantasy, Turn-Based, Action?

If turn-based fantasy is your thing, please look at D:OS.

Hard to say without knowing what sort of setting, cost structure, and gameplay you like.

Note that, similar to ToxicTom's point, there are lots of co-op style games that are good for two players as well that aren't, strictly speaking, MMOs.

Stendhal! It's simple enough and small enough that running your own server wouldn't be a strain.

Guild Wars 2 is a great game to play with others. It's fun to explore around in and do quests and events with a co-op partner. The first game doesn't have as many people playing as Guild Wars 2 has but it's still up and running if you wanna give that a try too, it's quite a bit different from it's sequel but just as fun, in my opinion, but Guild Wars 2 has a free-to-play mode that you can try out to see if you enjoy it and decide later if you want to buy two copies for yourself and your son.

Another RPG that you can play co-op is Torchlight 2. It's not an MMO but it's still a lot of fun, especially with a co-op partner. It's basically a Diablo clone; lots of loot and lots of randomised dungeons to roam around and fighting enemies for more loot and several neat classes to play as. Nothing too complex. Just a fun action/RPG. Hope that helps!