my exes best friend likes me?

This really comes down to four possibilities.

First branch: what your best friend would think of you dating his best friend (and he needs to consider the thought you'll have sex of some degree if he's not completely over you). Many guys simply CAN'T get their heads around the idea of a brother or best friend or even a good friend dating someone they've dated. CAN NOT do it. It's not that they're a**holes or cavemen. Many can can very reasonably discuss it even, but there is no budging on the subject at all. They simply can't work with it.

If he shows any signs of that (and give him the credit of actually asking him and letting him think on it a little rather than "looking for signs" or doing the thinking for him), then you have to choose: is his friendship more important to you than some guy you've got the hots for? Or is there a chance it might be deeper and the friend could maybe be "the one"? That's actually the same choice someone faces when considering cheating, is the moment, or series of moments more important than life with Johnny or Janie? Or is the person you'll cheat with maybe to become a replacement, something better than you have with the one you'd be cheating on? Consider carefully. And maybe that'd carry the answer with it. If he's just someone hot and fun, there are many others who are too. If there's something extra there, maybe, then this is one of the tough choices adults face.

And that last possibility might sway a CAN NOT kind of guy.

And the second branch is just that: is there really something special possible, special enough to risk losing the "best friendship" with your ex? If not, why risk that? If so, you have to don't you?

First step though, ASK your ex- about his thoughts on the subject. That's what best friends do for each other and their relationship. They give the other a chance before presenting them with "tough luck, it's done already."

Working with multiple factors on each side of a decision, like this one, is the only real step from childhood to adulthood. Getting the decision itself right is skill at being adult. Making it the right way, that's adulthood. You are going to spend the rest of your life being an adult and living with the results so handle this right. It'll be a good start and help with that skill side of things. Be pretty nice to treat your best friend right too, right?