boner?

My boyfriend and I were out today and we were sitting on the train when this girl came onto the bus. He kept discreetly looking at her and within the first minutes of her being on the train he picked up his back pack and put it on his crotch like he was hiding a boner. I pulled the bag of his lap to get some "gum" and he squirmed and pulled his pants away from his penis trying to disguise the mild boner I could see. As you can tell I am a very self conscious person. I haven't asked him or told him about this, but it's upset me all day. I just hate the thought of my boyfriend of 2 years looking and getting a hard on from another girl. Does anyone have any advice. Thanks x