Wow, this is like the dating version of an algebra problem. And hell no, he can't be trusted.
Dumping somebody for their best friend is a pretty lousy thing to do. Dumping somebody for your ex is a pretty lousy thing to do. Combine the two and you've...well, I have no idea what precisely you are, but I know it isn't trustworthy.
Your ex here doesn't know what he wants. That simple. Don't put yourself on the line until he does.
Hell the the McFreak'n NO!!! Why would the "ex" want him back anyway?!?