I agree with everyone above.

I think it's great that you help out with the rent and utilitize (and food, etc.), and I think you should, but why must you co-sign a lease? Your boyfriend can do that himself if he's not strapped for money, then if you do breakup (which you yourself even said that could be a possibility) he can kick you out and get another roommate. If that were to come to pass, it would be an easier situation all around, especially for him, other than needing to find a subletter.

I think it's best for people who aren't legally married, or in a legal partnership, to keep things separate, at least to the degree where splitting is easy. Don't co-sign a lease, don't share a bank account, don't co-sign on a car loan, etc. You may respond to my mention of you writing "if we break up" with a "No, we won't break up", but the fact is, it happens a lot even to people who think they're totally in love. If you are not ready to commit to marriage or a legal partnership that provides more protections, I'd consider my advice. Many people even rush into marriage without thinking about it very very seriously.

If your boyfriend breaks up with you over you not co-signing a lease, then I guess that would be for the best. Or, you could take care of the whole lease without your boyfriend co-signing.