Hai. Shoulder conditions are more commonly seen  in diabetic patients. As age progress the joint movements also gets affected. There are many special test to be done to check like if he has got frozen shoulder or shoulder impingement or any tear in rotator cuff muscles or any bone spur formation. -For time being apply hot water bag for 10-15mints multiple times a day. - proper rest - No lifting heavy weights -No overhead activities - And as far as possible avoid sleeping on the affected side. -Pendular exercises can be done just to maintain the mobility of the joint.      ->Ask dad to keep one foot in front and slightly bend forward. Hold a water bottle and then swing the arm forwards, sideways, clockwise and anticlockwise. This shouldn't cause any pain. Each movement 10 times twice daily. Note he shouldn't bend this elbow while doing it. It should be done slowly.

Can you tell us if he is able to lift above shoulder level using his other hand without any pain? If there is pain when lifting above shoulder level, it could frozen shoulder or rotator cuff involvement. If he doesn't have pain above the shoulder level, it could be deltoid weakness. In case of frozen shoulder, wax and mobilization would help. For weakness of deltoid, you need a physio to measure the strength grade and provide strengthening exercises accordingly.

Seems frozen shoulder. Physiotherapy treatment  for few days and stretching exercise or strength exercise  shoulder

We need to understand his full history first. Could you share his medical history and his daily routine? It could be a frozen shoulder by the way you are describing his condition.

Based on the history It could joint alignment issue With proper Physiotherapy evaluation & treatment Sometimes small belt could be required to align the joint

He need to start physiotherapy as his joint is restricted. 

He may have joint restrictions.. wax and mobilisation will help