Hi,
You have not described exactly what problems and symptoms your brother is having with his jaw except that he is unbale to open his mouth wide. This condition is known as trismus and can occurs due to scarring and fibrosis in the soft tissues which have been subjected to surgery or radiation. In some cases, involment of these tissues by the primary cancer can also contribute to the scarring after treatment. Since scar tissue is inelastic and has less blood supply, it can lead to contractures and difficulty in opening the mouth. Mouth opening excercises and devises to assist mouth opening can help in such cases. However, the improvement is generally slow and partial.
There is another entity called "osteoradionecrosis" of the jaw which can occur due to reduced blood supply to the mandible. This can occur if the jaw has received high doses of radiation and is more common in people with poor oral and dental hygiene and in persons with advanced cancers of the mouth. This condition is associated with repeated infections of the jaw which can lead to discharging sinuses from the mouth or skin over the jaw, loss of teeth and gradual destruction of the jaw. Depending on the severity, this condition can be managed either conservatively with antibiotics, debridement, hyberbaric oxygen therapy etc; or by surgically removing the necrosed and infected part of the jaw and replacing it with a bone graft or a metal prosthesis along with vascularized tissue flaps taken from another area of the body. This procedure is performed by a maxillofacial surgeon.
All the best