Your son’s behavior is quite typical of 7-year-old boys. The best defense against the influences in our children’s lives is a good offense. Talk with him about making good choices, and how good choices are helpful to him and bad choices are harmful to him. Keep it simple and meaningful to him.
Make certain that the examples you use involve choices he is facing and outcomes he would actually experience. Then role-play real-life situations in which he may find himself to see what choice he would make. If his choice is good, praise him and point out how he benefits from his good choice. If his choice is not good, talk with him about possible outcomes of that choice and how these outcomes specifically affect him.
A suitable situation to role-play with your son is that the most popular boy is doing something wrong and invites your son to join in or perhaps dares him to do something. How would your son handle that?
By doing activities like role-playing, you will better understand his thinking, and you will have the opportunity to teach in areas where he is not making good decisions.
