How old is your son? If he is over the age of 12, then then he can decide to live with one parent or the other. If he is under 12, then he cannot make that decision independently.
You will need to file a motion to change the visitation/custody arrangement. You and your ex-wife can agree on the change. However, if the two of you can not come to an agreement, you will have to persuade the court to make the change. The court will be looking to material and substantial change since the entry of the divorce decree and if changing the custody arrangment is in the best interest of your son.
Bottom line -- I would see a local attorney to discuss your options.
You need to file a petition to modify your divorce decree stating that that there has been a material and substantial change of circumstances and that it is in the best interest of your child for him to be primarily with you. If your son is at least 12, he can state a preference as to who he wants to be with primarily. Keep in mind that this is not the same as him "deciding" who he wants to live with because his stating a preference does not automatically mean that the court agrees that he should live primarily with you. However, it would be a significant factor in the decision, especially as he gets older. If he is under 12, you can still file the petition to modify, you will just need more evidence to establish there has been a significant change of circumstances and that it is in your son's best interest to be with you. Each of these cases are very fact specific, so I suggest you speak with an attorney experienced in handling these types of cases. I hope this answer helps.