It's understandable. When people compliment me I like seize up and end up muttering inaudible nothings lol. I've gotten better about saying Thanks now. It takes some practice!
Hmmm, how about this.....when he says something sweet again, like automatically say thank you. Or like push away those evil thoughts or voices in your head that try and persuade you he's lying. I know that's must easier said than done though.
You could try the straight up approach and just flat out tell him you have trust issues but you'd appreciate it if he was patient with you and doesn't take anything bad you say personally.
These are just suggestions. I dunno if they help but don't beat yourself up. People in this world are very selfish and rude so it's easy to get hurt.....but just don't give up cause there are good people out there willing to support you.