Helping jealous boyfriend

It would be really nice hearing other peoples opinions and experiences, my situation:

My boyfriend and me are together for one and a half years after meeting abroad two years ago.

He is so great, very caring and always insists to talk about everything going on in our relationship. I love him very very much and I never felt more comfortable with my life. He helps me achieve so much that makes me happy, like traveling together or beeing active. What i want to say is, that we have a really healthy and honest relationship.

The only problem is, that he gets very anxious/insecure about my past relationships.

About that:

I am a very not jealous person, so when we were starting to get to know each other I talked about them a bit, because they are just part of my life story. I had 2 relationships for 3,5 and 1,5 years and some situationships, looking back I would not want to date any of them anymore, but in that moment I did not know better and today I am kind of grateful that they all gave me good or bad experiences that I am now able to know what fits me. I am not in contact with any of them anymore.

He previously had just very long relationship for over 7 years, they started dating with like 13 haha. I don't have any problems with that what so ever, maybe just curious because I am really interested in his upbringing and teenage years where she is obviously a part of.

So he simply never dated anyone that had exes, and was as surprised as I was when he first felt that anxiousness. He is a very competitive person and always strives to be the best, and even though I never really talk about my exes when not ask he always compares himself in so many points.

For example we were playing basketball a week ago, I mentioned that I know a game for two players that we could try, he asked why I know a basketball game (I am not really into any sport that includes balls hahah) and I said that my ex liked basketball and we played that sometimes. Everything was fine, but at the end he really seemed like he was not enjoying himself. We talked about it a bit and the conclusio. he could give me was, that he was a bit out if shape ( he played a lot when younger and was really good) and did not want to look less capable then my ex when playing.

To make that clear, I really could not care less about basketball skills and never mentioned in any form that my ex was good or something.

I really had fun - until I realized that he had not and so in the end, although he was not angry at me or something and really tried his best to hide it, it kind of ruined our mood that afternoon. And thats what I am so annoyed about, it comes out of no where and ruins otherwise really happy moments.

I am slowly out of patience to act understanding, because I really am not able to and its not getting any better.

We always talk about it and he says he really tries to not be so affected by it anymore, but I don't see a change.

How can I help him be more confident? What can he try to improve?

What are your experiences with that, what did help you or your partner if any of you had this issue?