If your dad is at all wide and spends a lot of time with his hands close in front of him, e.g., mouse and keyboard, phone for hours on end, game controllers, electronics or model working, etc., have him get his shoulders x-rayed to check for developing bone spurs. I used to sleep on my side until excruciating pain began, but my previous doctor kept telling me he wanted to focus on other health issues first. When I told my current GP, she immediately checked and found bone spurs, and sent me to PT. At physical therapy, they said this type of issue is becoming increasingly common as people of the fatter generations are aging, and spending more time on electronics. They taught me some exercises and stretches and my pain is now more manageable.