Here is my two cents:
Dad would have to take you to court if you don't agree. That means six months or more before a judge makes a decision. Odds are that if your son has a normal teenage social life that once he gets back home and gets back into school life that he will want to finish his senior year with his class.
However, its also true that its time to look to the future, because once he turns 18 it WILL be his decision. If he doesn't have a good teenage social life, or if he has a girlfriend in dad's community, or anything else that makes life seem better at dad's (beside the lack of rules) you could honestly damage your future adult relationship with your son by refusing to allow him to live with dad. For example, if he is not popular at his current school, but has made lots of friends and is popular with the kids in dad's area, that could be a really huge thing for him.
Then there is the third possibility, and that is that he knows that dad wants him to live there, and he may either not really want to, but doesn't want to hurt his dad by saying so, or may be wishy washy on the idea but doesn't want to hurt his dad by saying so, and therefore NEEDS you to say no to the idea.
So...think it through mom. You know your son, we don't. Figure out what what is going on with your son and make a decision based on what you truly feel is best for him.
If there is any chance that you can spend some one on one time talking with son (face to face, not on the phone) prior to making any decision, that would be the best way to go.