You should NEVER give a prescription medication to someone for whom it hasn't been prescribed. It not only could be dangerous (say, suppose they have an allergy to it that they/you are not aware of), but it's also illegal to give someone a medication that has not been prescribed for them.
I would think the 4 advil would be ok for him, based on his weight, however, I would make sure he took them with food, as they can be very hard on the stomach.