If you won a judgment in small claims, then you would need to take action to enforce it. There are forms available on line for wage garnishment, judgment debtors exams, etc. Just check out the different forms available for use in small claims actions.
Were you and the other parents jointly and severally liable for the restitution, or were you each ordered to pay one half? You are liable only for the restitution you were ordered to pay. If you were ordered to pay one-half, you cannot recover from them because you paid their half as well. On the other hand, if you were jointly and severally liable, you may take them to small claims court.
I understand you paid the debt for which your son was liable. But you may recover contribution only for the debt for which you and the other parents were jointly and severally liable. Your son and his friend had other means of satisfying the debt.
By no means should you notify the Better Business Bureau. For one thing, this debt has nothing to do with his business. I don't know if a collection agency would act. Small claims court, which you mention in your post, is the best bet.