Dropped head syndrome as you just described it is a problem caused by a variety of neuromuscular disorders. Parkinson's is one of them. Others include myasthenia gravis, spinal muscular atrophy, and amyotrophic lateral sclerosis (ALS).
With Parkinson's, there is a movement disorder. The person hasn't lost the ability to move, but there is a problem with activating movement and loss of automatic movements. Everyday, automatic movements like turning, walking, or lifting the head diminish.
Freezing is another problem with Parkinson's. The person suddenly stops moving in the middle of an action. The foot gets stuck to the floor or the chin gets stuck on the chest. It's not clear exactly what causes this problem. There are many different theories. It occurs most often as the disease progresses suggesting degeneration of the neurologic system.
There is no cure for the movement disorders in people with Parkinson's. Physical therapy may help your brother learn strategies to improve movement. The therapist can teach him ways to improve smooth, coordinated movement. Sometimes something as simple as body rocking, clapping the hands, or jerking the eyes in one direction can help them get unstuck and moving again.