One thing you can do is think about the reverse situation. How do you feel about your exes? Obviously you loved them at some point, but you don't anymore. You care about your boyfriend now, not them. Your boyfriend has no reason to feel jealous of them. Well that's how your boyfriend likely also feels about his exes. Also, if his past relationships were so great, he'd still be in them. But he's with you now. He chose to be with you instead of them.