Don't offer all the details of your break-up to him. What he does need to know, however, is that you and his father do not love each other anymore and that you will never get back together again. That does not mean that you both do not love him or will not act with respect toward each other. This can be conveyed in an age-appropriate manner without condemning his father or giving him the whole truth behind your break-up. Perhaps he could better understand how you don't want to be with his dad if you compared it to something that has happened in his life. For example, if he's had good friends that he decided were not his good friends anymore, so he broke off his friendship with them.
Ideally, you and his father should attend a few therapy sessions together with a family therapist who has counseled couples in similar situations. Perhaps his dad could be persuaded that it's best for his son to come to an understanding about the finality of his relationship with you. A talented therapist can do this without making his dad feel like the "bad guy." Everyone needs to be saying the same words to him about the nature of your complicated relationship. Please get your son the help that he needs. If you don't seek help, your son will become more and more obsessed with the notion that he must get you back together again.