"Something strange has been poking at my butt since earlier." As soon as I had spoken, he turned bright red and fumbled around for an excuse

My junior Kiyoshi is hopelessly in love with me. He's a nice guy that's simple and fun to tease. But what was I supposed to do when he began rubbing his... thing, onto me when we were on a crowded train. Pissed, I wondered what would happen if I were to start playing with it.