The right thing to do is tell simply him you have a BF, and not get others involved, which would be embarrassing to both of you. The only reason to not tell/ask him yourself is because it's uncomfortable, but that's not a good enough reason to have others tell him.
If you are afraid he is actually just wanting to ride bikes and not hitting on you, then say to him straight out that you are not sure if he just wanted to ride or if he wanted to 'go out' with you. Explain to him your situation.

I guess it's ok to not call him and wait til you next see him and then if he tries again to get together- at that point you really should ask him about his 'intentions'.
There are way more tricky embarrassing things to ask a guy than this.

Still, it's sweet that he asked- either way! Good luck!