Tell your brother not to wait to have this checked.
May need blood work also.
Kidney stone pain can vary. Might still be that even if its a different kind of pain. I guess it COULD be his appendix but the right flank? Not exactly the right location for the appendix.
I've heard of liver pain presenting in the upper right part of the back. Does he do a lot of drinking or drugs by any chance?
Either way he definitely needs to get it checked out by a doctor. Might be Xray or MRI time.
He could look up the symptoms of chronic appendicitis, and see if he thinks there is a risk that is what is going on. Or he could go and get checked by a doctor.