Turning 18

Our teen is about to turn 18 in a couple of months. We've been doing TSG for about 6 months now and we've seen a lot of improvement. However, there's been a few occasions where this child has wanted to run away because "he's tired of our rules". This hasn't happened in a bit, but we're anticipating that he may resort to that once he turns 18. How would you handle something like that? I'm trying to preemptively prepare for a worst case scenario where he would want to go stay at his girlfriend's or generally do whatever he wants because "he's an adult". Thanks.