If you like his riding style and YOU'd like another riding buddy, then I'd do what CA suggested. Go riding with him and during the ride bring up your boyfriend.

If you have no interest in riding with him, then I'd just say "thanks for the invite, but I have a bf". If he says he just wanted to ride, then you can ask him to ride with both of you next time or you can tell him you don't think it's inappropriate (whether or do or not, it's a good excuse to avoid riding with him alone).