I don't think it matters that it's his mortgage or not. You'll be paying someone's mortgage if you were renting.
Some people recommend paying proportionally to your salaries as Amy8888 suggested.
If you think that is more fair because he makes $10K more than you, then suggest that to him and say you can't afford to pay the same % as you are doing now.
But the question about whether your boyfriend should be subsidizing your rent just because it's house is irrelevant. It's always going to be someone's mortgage! It's better that it's your boyfriend's rather than someone you don't know because if something breaks you got your landlord right there!