He doesn't have any options. You, on the other hand, can file a suit affecting the parent-child relationship and ask that you be named the conservator with the exclusive right to designate his primary residence. When you file your petition, you would file a motion for the judge to interview the child in chambers. Because your motion would touch on the issue of conservatorship, the judge would have to interview your son and see why he wants to move back.
Of course, you and your ex could always agree to let your son move back without the hassle and expense of a court proceeding, but I'm not sure what you've done about child support and that can only be modified by court order--whether it's the result of a contested case or an agreed order.
I agree completely with Mr. Daley. Until your son turns 18, he is required to live where the parent who has custody says. If he leaves without his dad's permission, he can be reported as a runaway and the police will bring him home (and can prosecute anyone who provides him with a place to stay.) If the dad agrees to allow him to come live with you, I would get something in writing.
Although I have answered the question to try to help you, you should consult with a lawyer in your area in person on the matter. In addition, my answer does not establish an attorney-client relationship between us.
Both answers above are correct. Perhaps the first thing you should try to do is sit down with your ex and attempt to reach an agreement. Often times, parents in his situation are reluctant to "give up" their child though. So, be prepared for some resistance. However, if you are able to clearly (and calmly) present your reasons why it would be beneficial for your child to move back with you, well then, hopefully your ex will open up to the idea.
If you are able to work out an agreement, then you should contact an attorney to assist you with formalizing the agreement into an order. If your ex will not agree, well at least you tried and now you know that you'll have to resort to litigation. Best of luck.
This answer is intended for informational and educational purposes only and should not be considered legal advice nor constituting an attorney-client relationship. This attorney is licensed in Texas.