Something as dramatic as you describe may be caused by an orthopedic or neurologic condition. A medical exam is needed to find out what might be causing the problem. X-rays or imaging tests will help identify any underlying cervical spine (neck) instability.
Weakness can be improved with an exercise program. Surgery may be needed if there is a spinal instability. Finding out what's causing the postural changes is needed first before a specific plan of action can be determined. Encourage him to see his doctor. Perhaps offering to go with him might help.