Case law is HUGE and in PA very poorly catalogued. Since custody for fathers is such a rare thing, you may want to start with state supreme court rulings and work down from there. Every state bar association publishes monthly reviews that contain higher court rulings and a summary of the arguments and these have proven to be my best resource.
The rulings are classified by legal branch which means there is a “family” law section. You may want to think about hiring a paralegal or a law student to do the research since they are inexpensive relative to lawyers.
The PA case was decided in 1992. The father and son argued that the custodial mother was failing in her parental duty by not supporting or encouraging the boy’s interests. She never attended any of his sports events (the father never missed one) and had no interest in the cars or sports teams that so captivated the boy.
She was, in short, ignoring his emotional needs and the court determined that the child could decide to live with his father if he wanted to and that no lower court had the authority to over rule the child’s decision.