D.B.
He needs to be told firmly and unequivocally that kissing is not allowed in school. I'm not sure he should be giving hugs, frankly - so I'm not sure you should encourage or allow that. He needs to follow the rule that we keep our hands to ourselves. That's one of the things that school is supposed to teach.
I think the teacher may need to separate him from the group when he crosses the boundary, and the bus driver needs to separate kids who are doing this, requiring them to sit in separate seats and not in front of/behind each other. They need to sit in their seats and not be jumping up either. Kissing on the bus is a safety issue - the bus hits a bump and some kid's head breaks another kid's tooth or nose, and then where are we?
I get that they are all copying each other and he's not the only one doing it, but they all need to stop. I would set aside school clothes that are appropriate for outdoor play and comfort and sitting on the floor, and not have him use his good clothes. You can also start the conversation that we don't dress up for our friends - that our friends judge us by our behavior and how we show respect for them and their bodies. You have to put that in age-appropriate vocabulary, but that's the message. At this age, people have FRIENDS, not girlfriends, period. The best way to make her dislike him is for him to force himself on her - that's a message he will need all through his school life including high school and college (and his professional life as well) - so we need to stop excusing our kids for this behavior and try to put it in no-nonsense terms they can understand.