Ima a girl and my bf got a bon er?

Hey im 16 and im a girl. When me and my bf were at the movies we kissed for the first time. He was wearing short jeans. We kissed and he suddenly stopped and walked away to the toilets.. I saw he had a stif fy .. what should i have done? I think he was embarrest, i didnt know what do to. Help? Incase it happens next time. Thanks