How to prove you didn't cheat ?

My boyfriend and I have been together 2 years & have had a really rocky relationship. I ended up moving out of state for two months and he got incarcerated. We talked everyday and i wrote him. I watched porn from time to time, never tried to meet anyone, talk to anyone, or anything along those lines. Every once in a while he will make statements about how i cheated on him while i was up there & no matter how many times i tell him i didn't he doesn't believe me. He keeps telling me to prove i didn't do anything, and i don't know what to do? How can you prove to someone you didn't have sex with anyone else? Its hurting my feelings so bad.