My ex never told any of his exes when he was in a new relationship, he never told me either but I found out he dumped me for another girl anyway. When it didn't work out with her, he just came straight back to me. Looking at his relationship patterns, he was almost never single because he just stayed with his girlfriends (even though he didn't want to be with them anymore) until he met his next girlfriend. That is what he did with his ex before me and then with me.
In my case, my ex would come back to me just to keep him company. But I think there are some guys out there who will go back to the same girl time after time because he does want to be with her and has always had feelings for her.
Every case is different, but unless the guy says he still has feelings for you and wants to make it work and actually tries to fix the problems there were before, don't take him back.