Remain active. Diabetes is a risk factor for frozen shoulder. You get pain, stiffness and decrease range of motion with this condition. Treatment is to use non-steroid antiinflammatory medications + physical therapy +/- steroid injections (be careful in diabetic, you might need to see ur doc to adjust dm regimen). Be patient, it takes a long time to see progress (2 weeks-months). Surgery also a treatment option.
Physical therapy. The initial therapy for frozen shoulder involves non-steroidal anti-inflammatories (your dad should clear this with his physician). There are home exercises such as the jackins exercises that he could do (3-4 times daily). It would be a good idea for him to be followed by physical theapist for this condition. If this does not work then he should see his physician for referral to an orthopedist.