Hi....
I think your dad having shoulder dysfunction syndrome(Periarthritis shoulder)...
It has 3 stages painful stage, freezing stage, thawing stage...especially his hand movement like inserting belt, combing hair back of head...
Physio treatment side
Pain management (hot water fermentation)
US therapy..
SWD therapy..
Then some ROM exercises like
Pendular exercises will help him for his shoulder pain and movement maintenance.
Next Steps
1) Muscle relaxing modalities(pain management)
2) ROM exercises
Health Tips
Pendular Exercises..
check his
sugar level,
thyroid etc..(if)