He really should put it on Steam for a couple of reasons. For one thing, he would be getting a lot more interest in the game, and surely more purchases. The more revenue he would gain would also in turn, allow for bigger, better updates, since the demand would be higher.
The only problem would be he'd have to take even more time with the updates, and possibly "polish" the game a bit more to make it feel better for the Steam customers, which would have to be better quality, and such.