I think that if you want an apology, you need to address the issues about his behavior and not just ship him off. If he is truly going to do better with his dad, then that's one thing, but please don't use this as a punishment to withhold your relationship from your son. He needs you. I see this in both my stepkids. Whatever happened on that trip, you need to address it. You need to sit with him face to face and find out what is going on in his head. Teenagers are miserable creatures sometimes, but they're also sensitive. Please go to your son. Talk to him. Sit down with him. Find a solution from the bottom. Behavior issues are just the iceburg you see. For example, my stepson used to come home and be very aggressive toward his dad. It wasn't that he hated his dad. It was that he felt so conflicted and missed his mom and didn't have the right outlet to express it. He had some heart to hearts with his dad and we also got him into football as a healthy way to express that pent-up emotion. You're in counseling? Call the counselor.
I don't know the details of your life, but I urge you to be the bigger person. He needs you to be his mom. I'd still want an apology later, but address the issue first. You've rejected him and he's hurt. It is very hard for boys to express that hurt. I think you need to reach out to the other adults for face time with your son. Even if he stays with his dad, you need to get this addressed, even without the upfront apology, IMO. You can still hold him accountable for his behavior, but it jsut sounds like there's so much else going on here.
Edit to add: Teens are very black and white. Friend of mine's DH gave his daughter an ultimatum and the kid took it. Now that kid is feeling like she made a mistake but can't take it back and the dad is too stubborn to relent in any way. I cautioned Friend that this is what would likely happen - a standoff. The girl is 14 and now has no contact with her dad because she doesn't know what to say and he's not backing down. What 14 yr old really understands the world? I think you can fix this, but you need to revisit some of the "don't blink" mentality, especially as he gets older. Sometimes we say, "Okay, so you want to go to this party? Why should we let you?" Or "You want to learn to drive? How can you show us you are ready for that responsibility?" We might still say no, but sometimes just talking about it makes the kid realize the whys and think things through that s/he didn't realize and sometimes we see things we didn't think about, either, and we can come to a compromise.