There could be a few possibilities as to why this child is being this affectionate with others.
One, it could be his personality. Some people are comfortable with touching others and expressing their feelings in a forward way. They will hug, kiss and tell someone they love them, even if it's considered inappropriate in the environment they live in.
Two, it could be that no one is setting boundaries for the child. Some parents do not believe in setting boundaries or are misinformed about how boundaries should be set for children. They may believe that a child is most loved if they are allowed to express themselves any way they see fit. Or they may not know how to set boundaries.
Three, he may be missing the affection he needs from his family. His family might not believe that being affectionate with a child is appropriate or he could be abused at home and has unmet needs, so he is reaching out to others in an attempt to get his needs met.
It is also important that this child have a balance of emotions. If he is always affectionate, happy and loving and never cries, gets upset or is discouraged, he may have something going on psychologically that needs addressed. It could be a personality issue or an emotional problem or even a brain chemical imbalance that is causing how he feels. But an evaluation would be necessary to pinpoint all the symptoms and narrow down what the cause should be. His school counselor or pediatrician can help with a referral to a child psychologist if it is necessary.
If there are no underlying psychological issues, then setting some boundaries with this child is important. While you don't want to suppress his affection for others, he does need to know that there are limits. It is unsafe for him to approach adults he does not know because they may hurt him. He also should be made aware of other people's reactions to his affectionate nature. He may need to be taught that he needs to ask permission to hug others or to touch them. Learning about personal space is important. He may frighten other kids or make them angry if he approaches them too quickly or does not respect their personal space.
I hope this has helped you,