I'm not sure about what you're trying to say. Are you saying that he says the ride turns him on, causes an erection, that sort of thing or is he trying to talk about how he feels and what his body is doing in an effort to understand.
If it's the latter, I suggest that because he trusts you and you've been sympathetic to his concerns in the past he is taking the opportunity while you're away from everybody to start a conversation. Since you're not comfortable talking about it, you can tell him, in a kind way, that you're not comfortable talking about this subject and suggest he bring it up with a man that he trusts. If you know of men that might be helpful, make some suggestions for who he could talk with.
If you think he's trying to tell you that he's attracted to you, I'd tell him, again in a kindly way, that this is not appropriate. I think it's good that his feelings are in the open where you can deal with them. It's really not that unusual for a pubescent boy to have a crush on an older woman. It's happened to me. I've told them that I was flattered but that the situation was inappropriate. I suggested that in time they would find a girl their own age to which they'd be attracted. In the meantime, I'm glad to be their friend but talk about sex or "love" was inappropriate. This has worked. I was still friendly with them and aware that they had a crush but they stopped making juvenile style overtures. If they did start to say something I'd just say something like remember what we talked about.
Keep in mind that your step-son is insecure and may have little knowledge about what is happening to his body. It also sounds like he isn't very socially aware. Does he act and talk inappropriately in other ways?
You don't want to hurt his feelings. You don't want to reject him. You do want him to learn that what he is saying to you is inappropriate and you want to say it in a loving way as a mother would. Be firm in a kind way.
I might also try to not spend time with him alone so that it easier for him to manage his feelings.
I think that you're just asking about his comment to you. But I also wonder about the way you react to the way he treats his father. It may help, if you don't already, to stand up for your husband and let your step-son know at the time he's misbehaving that you do not accept his behavior. I hope that you and your husband have decided on a consequence and enforce it each and every time.