Personally I'd rather pay more than my share to avoid paying less than my share. Luckily my main hunting buddy is the same way but I've encountered those that will take as much as they can get.
My advice would echo everyone else and recommend you talk to him about it. I think people fall into three categories when it comes to shared expenses. There are those who fight for the chance to pay, those that will pay exactly their share, and those that are looking to pay as little as possible. While I prefer the first group I don't have an issue with the second. It just requires a little more work getting everything listed and split evenly. You typically don't see many that fall into the third, but I avoid them like the plague.
A trip to the bar will typically reveal who is who. The guy who throws a $20 in the pot, the guy who puts $17.50, and the guy who says he didn't agree to pay for any of it. These distinctions also commonly reflect work ethic and division of labor as well.
One important note when using the bar tab method is remembering to account for an individuals means.
I'll never forget the time some college buddies of mine paid my beer tab. I was still in school and not making a lot of money at my college job. After I bought the fourth round I let them know that I was done as I couldn't afford to match any more rounds. They both responded by buying wayyy to many rounds for all three of us, saying that they had real jobs and that I didn't need to worry about it.