My ex is in a rebound relationship what do I do?

Ok so me and my ex were together for 5 years and lived together for 4 we were madly in love with each we were even engaged shared everything we even ended up liking things we didn't before. We were always together. The reason he broke up with me was because I got into an argument with his mom but I had a good reason for it she hated me. It's barely been a month and two weeks since the split. At first when he broke up with me i accepted it and began to do my own thing. he was going crazy he told me he regreted leaving me. he started acting very mean with me putting stuff on facebook and he even told me he is talking to someone else. im devestated but i have yet to contact him and i dont put anything on facebook or em i trying to get revenge. Now I think he's with the girl he told me he was talking too it's all over facebook pics and everything I'm crushed and we don't talk idk what to think is it a rebound???