He thinks I’m cheating on him from the moment we met but I haven’t, how do I prove it to him I haven’t or what should I do?

I’ve been with my boyfriend for the past year and a half. We met over the unfortunate passing of his brother, who was someone I cared deeply for and I was very close with. The reason I reached out to him in the first place was to see if he was ok and to maybe make a new friend. It turned into something more, and I didn’t expect it. From the start we have had issues bc he got into his head that I was cheating on him. He didn’t directly express it right away so I didn’t catch on. When I met him from the beginning he said he had trust issues and to not take offense so I tried not to. Even though some stuff he said to I was concerned with. He told me of past relationships and what they did (How they all cheated) and I chalked it up to well it’s bc of what they did so I won’t be accused since I wouldn’t anyway. He said things to me like “he knew right away” “he could tell it was obvious” “his instincts told him” “he couldn’t be more wrong and it’s extremely frustrating. I’ll admit when I first met him he made me super uncomfortable talking about anything with work bc the second time we hung out he expressed it bothered him I Tattoo all day and “spend so much time with clients Or other people “ And other weird shit so I kind of avoided some work talk which bc of it is which is why he thinks I was up to something 😑. if he didn’t say all these things that made me nervous I would have talked more freely but I didn’t feel comfortable. He thinks its at work with my boss. I work with him, his WIFE (also boss) and their apprentice. It’s maddening. Any time I’m not working I talk to him the ENTIRE TIME. I can explain some scenarios he deemed suspicious in comments. Besides all this, I’d also like to add I've never loved someone so intensely we have a very deep bond and he is my best friend and perfect to me in all other ways which he doesn't seem to believe sometimes. I’ve been loyal. I’ve never cheated on anyone I was with.
He thinks I’m cheating on him from the moment we met but I haven’t, how do I prove it to him I haven’t or what should I do?
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