Sorry you are having to deal with this. Here are some quick answers:
1. Every medical visit and prescription must be accurately documented in a medical chart. It is legal for her to administer medications essentially anywhere so long as the visit and prescription are accurately recorded. As a parent/guardian of a minor, you could request those charts. Unfortunately, your stepson is 19 and a legal adult. You can't control what happens to him medically unless he is found to be incompetent to make his own medical decisions. He may have consented to the medication.
2. A PA is not licensed to practice independently. Every prescription she writes is actually written under the license of her supervising MD. Every medical decision she makes is the responsibility of the supervising MD. Sounds like this MD is already aware that she is treating her own son outside of guidelines and hasn't remedied the situation. That's malpractice on his part. Now, if she is practicing independently without keeping her supervising MD appropriately aware, she is violating the terms of her license as well. Despite the fact that your son is legally independent, you can make complaints against these types of violations to the medical board in your state against the MD and the PA. The medical board can then investigate your claim and review your stepson's medical records. The complaint doesn't have to come from the patient themselves in this type of case.