Shoulder pain - lifting, raising arm, hands behind back, sleeping. Been going on for 3 mos. Rot cuf?

Could be. Pain at night, with overhead activities, pain with reaching out and behind back, weakness are all signs of rotator cuff problems. An MRI will help secure the diagnosis. Treatment depends on severity of symptoms and extent of rotator cuff damage.
Complex Joint. The shoulder is a complex joint, & many conditions/ injuries (rotator cuff injury, bursitits, tendonitis, labral pathology, impingement, etc) can cause pain/ weakness, especially when lifing the arm. Some injuries of the soulder occur due to acute injury and some are from overuse. I suggest he see an orthopedist for a complete exam.