I want to get my ex to dump his gf so I can get him back
Is there a reason you think that the two of you should be back together?
Because I'm still in love with him, I won't get another boyfriend.
That is a mean and spiteful thing to do. You shouldn't get involved in your ex's relationship. You need to give it time to find someone else and not just break up your ex's relationship.
If I don't do it, he won't come back.
I think that you should try to stay out of it. They are trying to move on and I think that it would be a good idea for you to do the same.