I think this sounds like typical, age-appropriate behavior, perhaps amped up a bit by some resentment that you moved out of state. Even big kids get upset over things like that, even if it's a totally logical decision and benefits everyone, the little boy in him might be feeling like you chose your job over him so screw you.
So at 18 and out of high school, what are his plans? If he's going off to college, most kids push their parents away during senior year and the summer after to prepare themselves for being on their own in college. If he's entering the workforce and doesn't currently have firm plans for further training and education, he may feel like he's in limbo and that it's time to cut the cord and be a man, and this is his way of doing so. Also, if his girlfriend lives at home with her parents and has an intact family, he may be drawn to that right now. If she doesn't live with her family, you've got a whole 'nother kind of trouble on your hands.
Anyway, I would get together with your ex and have a sit-down with your son about re-drawing boundary lines. You can respect his adulthood, independence and need for autonomy but he has to respect some rules about keeping in touch with you, coming home, etc. Presumably he still lives in your/your ex's house, eats your food, watches your TV, uses your water to shower, uses a cell phone that you pay for, etc. In that case, he still needs to live by house rules. And make sure he's using protection with his girlfriend.
It sounds like a typical phase and I bet when he matures a bit you'll find that you're back to your old strong relationship. Good luck!