Thanks for asking me to respond.
Frozen shoulders take about 18 months to resolve by themselves. Best would be to do exercises to prevent the shoulder from becoming stiff. A good physiotherapist can help with that. Your father needs pain meds if he has lot of pain. Sometimes pain may develop or prevent him from doing exercises. He will need meds for such situations as well. You will have to visit an ortho surgeon who can examine the shoulder and prescribe meds.
I cannot prescribe meds as Practo prohibits me from doing so.