When I read this my first concern was a surprise pregnancy, but others seemed to cover this and you also said you've discussed this with him. I have 2 sons and we taught them the biology at a young age including how we are designed to procreate and if you don't actively work to prevent it IT WILL HAPPEN. Later we got into not having unprotected sex unless you are trying to get her pregnant. Believe it or not, there are young people who think you can't get pregnant the first time or if you only have sex occasionally.
My other concern is that he's sneaking out, behaving like he's doing something he has to hide. You seem to be very reasonable in your rules and expectations, yet he leaves through the window and spends the rest of the night out in the car. I'd be concerned that he feels he has to hide what he's doing. Because it sounds like if he just told you he's leaving and will be back at a reasonable time for a school night that you'd be ok with it.
An 18 year old wanting to be with his GF sounds pretty normal to me. If they are having sex that's their responsibility and they should be very aware of this at 18 and 19. At 18 he's entitled to have a personal life.
If he's not drinking or drugging then I'd be most concerned about the sneaking behavior. That's what a 13 or 14 year old does. It's kind of not necessary at 18.
My other concern is that he's sneaking out, behaving like he's doing something he has to hide. You seem to be very reasonable in your rules and expectations, yet he leaves through the window and spends the rest of the night out in the car. I'd be concerned that he feels he has to hide what he's doing. Because it sounds like if he just told you he's leaving and will be back at a reasonable time for a school night that you'd be ok with it.
An 18 year old wanting to be with his GF sounds pretty normal to me. If they are having sex that's their responsibility and they should be very aware of this at 18 and 19. At 18 he's entitled to have a personal life.
If he's not drinking or drugging then I'd be most concerned about the sneaking behavior. That's what a 13 or 14 year old does. It's kind of not necessary at 18.