I've lived with a boyfriend in an apartment before. We just split the rent in half. We actually made the same amount of money so that worked for us.
My husband owned his house when we got married (and soon after I was put on as a co-owner).
I've heard of different ways to split the cost of living. For example, you could determine how much you each bring home...so if total take-home is 50K and he makes 30K and you make 20K, then you would pay 40% of the bills (something like that. You can search for ideas on how to do that).
But I agree that it seems kind of sketchy that you're paying toward a mortgage but you're not an owner. It sounds more like he's your landlord, not a boyfriend. But I get that you want to contribute...maybe if he paid the mortgage and you paid the other bills? I don't know, it's a tough situation.