Most of Jon's confusing thoughts about the whole thing get pushed back by concern at the tone of Martin's voice. He isn't completely daft, the fact Martin was quite possibly on the brink of tears was pretty apparent to him. He has no idea what to say to help, especially given the surroundings, so he gives in to instinct and squeezes Martin's hand slightly.

"We're teenagers, they should expect this." He answers, though he lowers his tone all the more and mostly seems to settle back to watch.

More spoilers: his attention was not really on the movie anymore.