J.A.
Think of this as practice for the rest of his life. As parents we really want it to be someone elses fault when our child misbehaves, but the reality is, it's our child who is choosing to do so and that is how you have to deal with it. I know most children go through phases of imitating friends bad behaviors, all of mine have. You have to teach your son what is acceptable and what is not, and how to make good choices vs poor choices for himself. You are not alone in this, my daughter is four and going through it right now. Hang in there, teach him to be responsible for his behavior no matter what his friends are doing, and you will be fine (not to mention you will save major grief in the teen years if he understands these lessons at a young age).