Not knowing how old this young man is makes the response a bit more difficult. Regardless of his age, this is clearly a boundary issue and can cause him relationship problems now and in the future.
When children are small they learn physical boundaries by watching others and being around their family. The information they gain is that it is okay to sit on someone’s lap, hug them and even kiss them if they are part of the family such as a grandparent, aunt, uncle or cousin. As they get older they observe friends who hug one another when greeting. They eventually learn the dangers of strangers getting close to them and how this can be hurtful.
All of this learning helps them to define their physical boundaries. This is the personal space that you keep between yourself and others. These boundaries protect your body. You decide who can touch you, how they can touch you and so on. Physical boundaries include sexual areas.
If your son has not learned good or clearly defined boundaries, then he needs to be purposefully taught this information.
You could begin this teaching by having him help you list the people that it is appropriate or safe to hug, to kiss, and to sit on their lap or have them sit on his. In this exercise, you may discover that his comfort zones do not match your expectations. Explain to him a more appropriate boundaries and why it is considered safer or more appropriate. Help him understand the problems that will come from not respecting others boundaries or requiring others to respect his.
If this kissing behavior is occurring at school, communicate with the school about what they are doing as a result of the behavior. Support the consequences they are using and implement your own consequences at home each time you receive a report that this has happened. If there is no violation, reinforce that with praise.
Teach to boundaries frequently. When in a social setting other than school, quiz him about what behaviors would be appropriate with individuals present in that setting.