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.

I would suggest asking him how he intends to handle this. I would think that he will know how to handle it. But if he comes to you for advice, if I were you, I would simply advise him to treat all his friends the same. Personally, I think you don't have to worry too much as children this age still does not have the concept of romantic love. The teasing is probably harmless and would go away soon. I think it is a great opportunity for you to see how your boy handles such interactions. Will let you know more about his personality.