1. It's unlikely that, in any state I know, it's statutory rape for an 18 or 19 year-old boy to have sex with a 16 or 17 year-old girl. Typically, statutory rape statutes require a minimum age difference (usually more than 3 years) once one of the partners is 15 or 16. So, much as some people would love to use this argument, it's probably not true.
2. I can't imagine any circumstances under which a boyfriend's parents should have anything to say about his girlfriend's choice of birth control method. That's waaaaaaaay over the line, and might earn you a punch in the nose from me if the girl happened to be my daughter and I happened to find out about it. Which isn't to say that your concerns aren't rational -- of course they are -- but it really is none of your business. Discuss your son's responsibilities and the importance of protecting himself with your son. Do not tell him to make his girlfriend go on the pill. He can no doubt think of that himself.