Boyfriends cheating

My boyfriend and I decided to go on a trip down to Texas to see his family that he hasn't seen in 10 years plus. The ride is about ten hours and on the first day there I saw a snapchat screenshot of a girl telling him to send her pictures. I got upset but I didn't want to ruin the week so I decided to leave it be until we return. Tomorrow is our last day on the trip and I was looking at something on his phone and the app kik was open. He has been posting pictures of himself and getting pictures back from several different people. We have a ride back that's 10 hours so I really don't want to say something now but should I? If I do it now I'll have to spend the whole car ride back with him. What should I do?