Was he masturbating to me over web cam?

This guy I know from school and I were video chatting the other night & after a while I noticed his arm was moving back and forth kinda fast while his hand was most likely down his pants since he kept looking down. I heard a squeaking sound too and he was kind of biting his lip. This went on for like 30 seconds. I umed he was masturbating, but I wasn't sure what to. He apparently tells his friends that "he'd do me any day", so does that mean he was jerking off to me over web cam? Also, why do guys like to do that in front of girls? :/