I think you need to lay some ground rules down with your boyfriend. I'm sure you can understand why he's upset with your ex-boyfriend, but that isn't his battle to fight. You aren't with your ex-boyfriend anymore (and shortly, he won't even be in your life anymore as a friend), so I think you should tell your boyfriend to just drop it. If he wants to comfort you about past problems with your ex-boyfriend, that's fine, but that should be the extent of it.

You say you can't convince him to leave it alone, but you must keep pressing it. If he really cares about you, he'll drop it for your sake, because he cares about your feelings. Let him know how upsetting their arguing is to you and hopefully, he'll come to his senses and just be there for you as your boyfriend and not as your white knight.