Thank you for trusting me with your question. I recommend you be bold. Confront your boyfriend with his projection. You said it very clearly here, now it’s time to say it clearly to him. Let him know that because of what his ex-wife did he is interpreting everything as if he’s going to be betrayed again. Explain that it is not fair to you because it means that, in spite of what you do, say, or share with him, his mind interprets it as a betrayal. This leaves you helpless. The pain his wife has caused him in the past is never-ending in his future. Be clear about the fact that this is not yours to fix. Any woman he dates is going to be seen as a cheater because in his mind he can only foresee it happening again. Here’s a link to an excellent article about this kind of trauma.
Tell him that it feels miserable to be mistrusted when nothing you’ve done warrants it. I would be very clear about this putting all the responsibility for his perception and struggle in the relationship on him. Don’t mince words.
I know these are strong terms. But the bottom line is he either changes his attitude and sees you for who you truly are or there is no need for you to be with him.
Finally, offer to go with him to see a couples therapist. This can be both supportive and enlightening for him. This gives him a chance to hear what you have told him from a neutral third party, get support from you by going with him, and give him a chance to see the dynamics he is causing.