I think he’s slightly lying... regardless I’d say you’re an amazing girlfriend! You’ve actually gone out of your way to ‘heal’ this thing you see as potentially harmful/getting in the way of your relationship.

Don’t do something you don’t feel comfortable with!

If you want to ‘get past’ it, I’d say try oral sex but mutually - so in the 69 position. That way, your mind will wander and be consumed by something other than your monotonous one way pleasure giving! Try think dirty thoughts, occupy that space in your head. Get into the moment.

This is something that will likely pass in time! I’d keep telling yourself that your boyfriend is ‘yours’ now, he isn’t going anywhere and that you love each other (if you do). Maybe solidify how you feel about each other may strengthen the bond, and then make topics like his past ‘encounters’ less important.