As terrible as the vomit cycle syndrome is, it is treatable and requires some education. Your son should see a doctor not a felon. When he is not on a cycle, there are medications that can be prescribed to help him control his symptoms and a physician is best qualified to help him through that.
People generally improve after they have learned how to control the symptoms, so this is a process. You can encourage your son to put his trust in a medical professional, not a pusher. Heroin is never a good idea, never a good choice and never an alternative for legitimate treatment.
Part of me thinks the CVS is his excuse to use heroin. I know that may sound harsh, but there's a lot going on here. Your son needs help.