In order to change custody, you must have a substantial change in circumstances. Your situation might qualify for a substantial change in circumstances, but most lawyers will want more information. For example, how is your son doing in school and how old is your son? I would recommend meeting with an experienced child custody lawyer to discuss your particular set of facts, even if it is just for a paid consultation.
I would also suggest that when you make an appointment with an attorney for a paid consultation, make sure the attorney is one who regularly practices in the county in which you and your ex-husband live so that you can get a better idea of what that sort of litigation will entail in your county.
Legal disclaimer: The above answer is a general explanation of legal rights and procedures. It does not constitute legal advice. Nor does it establish an attorney-client relationship between the individual posting the question and the attorney providing the answer.
My office handles cases in your county.
To change custody or visitation, one must show a material change of circumstances. Depending on the original paperwork and details we do not have, you may be able to make that showing.
The costs and difficulty and likelihood of success depend on details I don't yet know.
Please feel free to call me with details.
Following up on what the other attorneys have said, all of which is correct, I think its important to note that in the state of Florida for example, one must show a substantial, permanent, material and unforseen change in circumstances warranting modification of an existing custody order. In Florida, while the term "custody" is no longer used, older judgments are still subject to modifications of "custody" from "joint custody" to "primary residential custody", or in more extreme circumstances, where perhaps all overnight timesharing is terminated, to "sole custody". It's important to consider whether the change in hours is permanent in nature, or will revert back to the previous shift schedule, and perhaps an easier resolution would be to seek a modification which requests a "right of first refusal", to wit: if either parent is going to be away from the chidlren for more than __ hours, then the visiting parent shall give the other parent the first right to have the children, and only upon refusal, may he or she leave the children with a third party, such as a girlfriend. This may be easier to accomplish than "sole custody".
I think the court will look to the reasons why your son does not want to stay with your Ex-Husband's new girlfriend (if this is a basis for modification, and is predicated upon something other than a desire to be with you if you are seeking to prohibit his girlfriend from ever watching the children).
I don't believe it hurts him that he moved so close to you, and in fact, seems to be a great reason for adding a "right of first refusal" and perhaps using his relocation and change of work hours as the basis for modification to include such a provision. This would in all likelihood be cheaper and less emotionally taxing than a full blown custody battle.
Good luck. Please consult local counsel to determine the types of "custody" where you reside, and the basis for modification.
Sign up to receive a 3-part series of useful information and advice about child custody law.