a couple of thoughts -
1) you have no choice but to trust him in what he says to you. Maybe he does like her like that, maybe not. People can change their minds, and that is OK so long as they inform SOs of these changes if they are relevant to them. Maybe he is just curious... who knows.
I think you should hang on for the ride. Let him work this out in his head. This is a great time for you to get a "taste of your own medicine" and not in a bad way - It will only help you understand each other better.
2) I am a musician. I like to play music with people. Sometimes I have close friends that know this about me and want to join in. For me - Music with people is like chemistry. It cannot be forced. Even if my BFF played an instrument that my band had been looking for for ages, I would not necessarily feel the vibe with her and want her in my band. It is a personal thing, but shouldn't be taken personally. It doesn't mean I don't still love her and that she will lose BFF status. I am not saying that is what is happening here, but it could be.