I'd call the pharmacist first and get the indications for half a dose and age/amount scale. First, I might try a prescription strength dose (800mg) of the Motrin, then if that didn't work, ask the pharmacist if the two could be given together. I would not just self prescribe to a 15-year old but I would definitely do what is necessary to make my child comfortable, within safe limits of course.
No judgment...I don't blame you for wanting to help your child with pain. I'm not one of those people afraid of pain meds, but I would want to do it and stay within safe dosing limits. One or two doses will not harm or cause addiction issues, but I would still reach out to the pharmacist for advice. 4951