You're still with him after 13 years. Obviously you think he's cool enough. If you didn't leave him for someone cooler when you could only get him or someone else, why would you leave him for someone cooler now that you could have both? That would end up in a loss, the loss of him, and no gain, since you can already be with the other guy anyway. That would be counter-productive.
Of course, that kind of thinking probably doesn't help his insecurities. But the bottom line is, it's toxic thinking to think in term of "who is better". People are different, and you like different things about different people. They're unique, and special, and you don't want to replace him with someone else. You have 13 years of positive history, and he needs to dig into that to trust you. It seems that he thinks you only stays with him because you didn't realise other people were better, and the second you're made aware of it you'll go. This is BS.
You stayed with him because you love him and you want to be with him, and he's the best at being him. And all of these things are going to remain true even if you're with somebody else. That somebody else might be "better" in a bunch of things, but he'll never be "better" at being the man you've been with for 13 years, the man who's helped raise your kids.