There is an ex of mine (from 4 yrs ago, not very serious) who emailed me the other day saying he had broken up

Well it would seem as if, you are the rebound. You just answered your own question....he's a jerk, he's looking for someone to help him get over his girlfriend who is more likely to get back with...so you be very careful....You don't ever want to be the rebound, it's not a good feeling.