When I was young I put aside/broke up with a really wonderful guy who had jealousy issues, now that I'm older, I realize that we had a special connection, and he was so jealous because he was afraid. And we were both really young and immature.
I wasn't doing anything for him to be jealous of, it was in his own head. What could I have done differently so we could have matured together, and gotten together when we were both ready?
I think I could have made a hard boundary about it. Something like, I understand what you're saying even though what you're thinking isn't true. I want you to understand me too, I really like you. And I want to be with YOU.
But this jealousy is really toxic, and taking all the fun out of our time together. Can you try not to be afraid that I'm going to leave you , or find someone else? If you stay with me, and stop being afraid all the time, you'll see that I'm not like that. And that I will stay with you.
The unspoken part of that is, if you keep acting like this, I will have to leave.
But, most people react to an ultimatum with defensiveness.
If he thinks it was his own idea, he'll probably go along with it better.
Good luck with this. I think young love is a once in a lifetime thing, and should be cherished.