My young son is in lower primary and popular with girls classmates. His peers tease him with those girls classmates are bf/gf stuff like that. He was annoyed and puzzled. How do I explain to him that it is too young to get involve with this while not destroying his friendship with the girls in school.

Encourage him to put friendship first -- be it with the boys or girls in his class. Don't discourage him to stay away from girls because this will not be good for his future social skills. Tell him that it's okay to have "girl friends" (not girlfriends) and encourage him to have as many friends as he wants. Hopefully he grows up to appreciate the value of friendship and use that as a foundation when he is ready to have actual BGR.