Hi and welcome to HCM.
This is reffered pain caused by phrenic nerve irritation
due to enlarged spleen
and raises intraabdominal pressure. It wont stop until spleed is enlarged. Spllen can be removed,but it may be risky in such atients. There some potent painkillers wich might help based on opiats. He should consult with his doctor about it. It is had to expect that nerve will die or stop pain producing unfortunately.Tratmen of cause will make all symptoms easier.
WIsh you good health.