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.