If you have to ask, you probably need a lawyer. You should always have a lawyer when entering a custody battle, which this might be. Children can speak to the judge regarding their preferences at 12 years of age, but the judge no longer must abide by that request. Therefore, you would need a hearing in addition to his request showing why your place is better for your son.
Another option is just to talk it over with the ex and discuss doing a trial period, say a school semester, to see how things go. Get everything in writing, and understand that child support is going to be an issue if it is ordered. Only a court-order will negate child support or abate it, but if you get an agreement in writing stating your son will be living with you and that child support should be stopped during that time, most of the time a Court will respect that if the child support issue comes up later.
If your ex agrees to a permanent change, you need to get a modification of the child custody and support orders in place. If it is all agreed, you could get an attorney to draft the pleadings and order for a flat fee, perhaps.
If it is going to be contested, get ready to lawyer up.
This answer is intended for informational and educational purposes only and should not be considered legal advice nor forming the attorney client relationship. This attorney is licensed in Texas.
As noted above, your son's preference is allowed to be voiced to the court, but that alone will not guarantee the court awarding you with primary possession. You should consult with an attorney to deteremine, based on your specific facts, the best route to take to accomplish your goals. Best of luck.