Will the mother consent?
OK, then he's in for a battle.
Clearly, the court determines custodial/visitation issues based upon the axiom "best interests of the child".
So, your son will have to file a motion for modication of custody in teh court where the current order is.
In that motion he will have to request a change of primary custody based upon that it's in the best interests of the child(ren), that they desire such, and all other reasons that he can think of.
The court will schedule a hearing - a mini trial on the matter. Yes, the children can state their preferences as to whom they want to live with. The 14 year old is most likely old enough to potentially make a decision but not the 10 year old. However, the 10 year old can substantiate events that occurred in her house, etc.
What happens in these cases is that the judge takes the child(ren) back into his chambers (the judge's office) one-on-one, beyond the eyes and ears of the parents and other people in the court and discusses the matter at length with the child(ren). They talk about many things while in chambers, the current custody situation, the parents, if a parent is pressing them to say something or lie to the court, what problems if any they are having where they reside now, school, sports, etc.
After talking with the child, the judge then makes a determination of the child is mature enough (not old enough) to make an intelligent and informed decision based upon the facts and situation. If the judge determines that they are - then the judge will follow the child's wishes. If not, then the judge will do what he/she thinks best under the circumstances.
Realize that your son has the harder burden to change custody because she is the status quo - that he's going to have to point out problems and how he would fix such.
Your son has to do such.
Yes, special circumstances would be certainly relevant to the court in making their decision as to custody.
What are the special circumstances?
DISCLAIMER: Answers from Experts on JustAnswer are not substitutes for the advice of an attorney. JustAnswer is a public forum and questions and responses are not private or confidential or protected by the attorney-client privilege. The Expert above is not your attorney, and the response above is not legal advice. You should not read this response to propose specific action or address specific circumstances, but only to give you a sense of general principles of law that might affect the situation you describe. Application of these general principles to particular circumstances must be done by a lawyer who has spoken with you in confidence, learned all relevant information, and explored various options. Before acting on these general principles, you should hire a lawyer licensed to practice law in the jurisdiction to which your question pertains.
The responses above are from individual Experts, not JustAnswer. The site and services are provided “as is”. To view the verified credential of an Expert, click on the “Verified” symbol in the Expert’s profile. This site is not for emergency questions which should be directed immediately by telephone or in-person to qualified professionals. Please carefully read the Terms of Service (last updated February 8, 2012).