If they fight, then BOTH are immature jerks and you need to dump the new one & stay away from the old one. As an EX, the 1st guy has no RIGHT to be upset about anything you do or anyone you see. If he's "fuming", he can trot his behind to the gym or basketball court & work his anger out. And the new guy-all HE has to do is walk away from the ex if confronted. Fighting over a female-its silly when you're kids & silly when you grow into adults & do it. But then again, what can you do to STOP a future fight? Nothing. If they want to act like fools, let them. Males operate more on testosterone than intelligence!
Well i think you should tell your ex he already had his chance and that he should leave you alone and stay out of your life. As for the boy who likes you if you like him then you two should go together forget about your ex let him see you aint thinking about him and that you moved on. :) Good luck