If my son got caught selling weed in school campus he got arrested, what could happen to him now that he was arrested?

He could be taken to the county jail or the juvenile detention facility in your county. I cannot say which because you did not give his age. He might get charged with trafficking in marijuana, and that is a serious felony. It is also a serious felony if he is only charged with sale and delivery. He needs an attorney, so get one for him.