Forward head, dropped head, or stooped posture can occur as we get older. Gravity, weakness, and fatigue pull us forward into a more flexed posture. Many times these postures can be prevented with exercise and conscious attempts to maintain an upright position.
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.