If the idea that GF is "controlling" and is to blame for "cutting him off" is based entirely on his own use of the phrase, "She owns me" -- you are reading a lot into what may have been a tossed-off statement on his part.
If you have plenty of other reasons to think she is genuinely controlling that would be different, but you don't mention any, other than the fact that she "wants him to move out of state and he is struggling" with that. Has he said he is struggling? Has he said that he does not want to move? Is the move entirely to further her career or to be near her family solely? Or does he derive some benefit from the move, such as better job opportunities?
There may be a lot more to this story. You might have a lot more evidence that you didn't put in the post -- or you might be basing everything on a few words of his and the fact that she initiated the idea of a move that you dislike but he isn't resisting.
If you are going just on what you present here, there's nothing for you to do or say; that's not evidence she's a controller. Even if she is, he not new to adulthood and is making his own choices. If you feel he is actually being abused -- yes, men can be abused verbally, mentally and even physically -- that is different altogether, but if you just feel he has pulled away from you since meeting this woman, that is not necessarily something you can intervene in or fix.
I would keep the lines of communication with him wide open even after his move. Encourage his siblings if any to contact him and his friends -- not to barrage him with calls but to maintain good contact and never to complain about GF but to listen to him carefully and make sure he knows his family and friends are going to be there to listen and are interested iin him wherever he is. If he needs help later, if it turns out this really is a controlling relationship, he needs to know people will have his back then, but talking down the GF or the move at this point could backfire and cause him to defend her and cut off contact even more.