This is a popular topic around the PF blogosphere! Google around a bit and you will get many opinions and different ways to split things.
I agree with Mozzy that the best thing to do is to charge you a fair market rate for the amount of the house that you use. You didn't, however, mention how big the house is. Generally, renting is more expensive month-to-month than buying (hence why people are able to rent out their houses) so paying only 50% of the mortgage/taxes/insurance and expenses may be LESS than what your space would rent for (assuming he's not asking you to help him pay down the mortgage faster). However, that only applies if it is a small house. If there is a lot of space that you (individually) don't use, 50% of the mortgage/taxes/insurance would be too much.
So just decide how much space is "yours" and come to an agreement on what that space would rent for using comparable rentals in your area, and pay that plus 50% of the utilities. If there is a ton of unused space in the house and your BF needs more help with the bills, get another roommate.
I lost 40 lbs between Nov 2010 and Nov 2011 doing low-sugar for my "50 Pounds in 50 Weeks" project. I bounced halfway back up so I'm back for more!
I blog about personal finance at Evolving PF