Boyfriend is still Talking to other girls

I really thought maybe I wouldn’t have to come on here anymore because my relationship has been doing so well yet here I am. I was with my boyfriend last night and he smokes and I told him I didn’t want him to smoke in my car since I just bought it. The first time he listened but the second time he insisted to let him smoke through the window/door. However, to try to act funny he strayed blowing smoke at my face, into the car. I grabbed his phone as a joke (I AM NOT THE TYPE TO LOOK THROUGH HIS PHONE) to get him out of the car. Like a if you want your phone back, go smoke outside. He kept being stubborn so I swiped on his phone where it shows you just some previews but you’re really not in his phone because you haven’t put the password in. Anyways, my intention was not to snoop around or find anything but then I swiped I saw his Snapchat best friends list and they were all girls. Every single one of them. They are not his friends, I know his friends. They are just other girls he’s clearly still talking to. Is this cheating? Am I overreacting? Is there even a way to come back from this because how can I trust him?