I've seen all kinds of relationships in my six decades, including life-long deep friendships between former spouses. Some are healthy and some are not. From what you describe, your bf's relationship with his ex sounds pretty healthy. They divorced, will probably never want to live together again, and yet are fast friends who understand each other well. This is less likely to put stress on newer relationships for either party than a state of constant acrimony with the ex.
It also sounds pretty healthy to me that he won't be owned or controlled by another person. I don't see that as selfish. To me, that would actually indicate he could make a good life partner, but I don't have issues with jealousy, and I recognize that some people do.
So it's up to you. If you have feelings of insecurity or jealousy and are constantly in a state of unhappiness over this, then you would probably be happier to find a different boyfriend. And he would probably be happier to find another girlfriend.