I am sorry to learn about this disabling complication suffered by your husband after his brain injury.
The term heterotopic ossification refers to the new bone that is formed in injured tissues that should remain supple - ligaments, muscle tendons and muscles themselves. When I was a medical student (oh, so long ago!) the term myositis ossificans was used. This term was discarded as it implies inflammation - not a feature of such new bone formation. Heterotopic ossification can occur after any injury and has also been seen after surgery on bones or joints.
It is believed that injury provokes dormant stem cells in muscles, tendons or ligaments to produce new bone. The formation of this unwanted bone is especially likely if the patient already suffers from disease that produces unwanted bone. An example is ankylosing spondylitis, where new bone is laid down such that the separate vertebrae constituting the spine fuse into a single bone resembling a bamboo.
It is unfortunate that we have no simple method for preventing such new bone formation or getting rid of it once it has developed.
Simple preventive measures include gentle physiotherapy that exercises damaged muscles and joints and keeps them supple without aggravating the damage.
More complex treatment includes the use of corticosteroids and/or etidronate. Severe cases have been treated with radiation therapy and even surgical removal of the unwanted bone but the
results of such drastic therapy have not been encouraging. I am sorry at not being able to give you a simple, encouraging answer. I suggest you consult a senior orthopaedic surgeon in a medical college near your home for help and guidance.