Given the time frame between the time served in Vietnam and when he was diagnosed the possibility is low to none.
I'm sorry for your loss.
There is an information website for vets here - http://www.vietnow.com/pagesvaret/agent.htm
and they list Brain tumors as not related to Agent Orange exposure. There is another resource information site here -
http://www.2ndbattalion94thartillery.com/Chas/braincancer.htm
which lists some more information as well. And lastly, the VA also has information:
http://www.publichealth.va.gov/exposures/agentorange/ and they list the diseases related to AO exposure:
"Veterans may be eligible for disability compensation and health care benefits for diseases that VA has recognized as related to exposure to Agent Orange or other herbicides.
Surviving spouses, dependent children and dependent parents of Veterans who were exposed to Agent Orange and died as the result of diseases related to Agent Orange exposure may be eligible for survivors' benefits.
Acute and Subacute Peripheral Neuropathy
A nervous system condition that causes numbness, tingling, and motor weakness. Under VA's rating regulations, it must be at least 10% disabling within 1 year of exposure to herbicides and resolve within 2 years after the date it began.
AL Amyloidosis
A rare disease caused when an abnormal protein, amyloid, enters tissues or organs.
Chloracne (or Similar Acneform Disease)
A skin condition that occurs soon after exposure to chemicals and looks like common forms of acne seen in teenagers. Under VA's rating regulations, chloracne (or other acneform disease similar to chloracne) must be at least 10% disabling within 1 year of exposure to herbicides.
Chronic B-cell Leukemias
A type of cancer which affects white blood cells. VA's regulation recognizing all chronic B-cell leukemias as related to exposure to herbicides took effect on October 30, 2010.
Diabetes Mellitus (Type 2)
A disease characterized by high blood sugar levels resulting from the body’s inability to respond properly to the hormone insulin.
Hodgkin’s Disease
A malignant lymphoma (cancer) characterized by progressive enlargement of the lymph nodes, liver, and spleen, and by progressive anemia.
Ischemic Heart Disease
A disease characterized by a reduced supply of blood to the heart, that leads to chest pain. VA's regulation recognizing ischemic heart disease as related to exposure to herbicides took effect on October 30, 2010.
Multiple Myeloma
A cancer of plasma cells, a type of white blood cell in bone marrow.
Non-Hodgkin’s Lymphoma
A group of cancers that affect the lymph glands and other lymphatic tissue.
Parkinson’s Disease
A progressive disorder of the nervous system that affects muscle movement. VA's regulation recognizing Parkinson's disease as related to exposure to herbicides took effect on October 30, 2010.
Porphyria Cutanea Tarda
A disorder characterized by liver dysfunction and by thinning and blistering of the skin in sun-exposed areas. Under VA's rating regulations, it must be at least 10% disabling within 1 year of exposure to herbicides.
Prostate Cancer
Cancer of the prostate; one of the most common cancers among men.
Respiratory Cancers
Cancers of the lung, larynx, trachea, and bronchus.
Soft Tissue Sarcoma (other than Osteosarcoma, Chondrosarcoma, Kaposi’s sarcoma, or Mesothelioma)
A group of different types of cancers in body tissues such as muscle, fat, blood and lymph vessels, and connective tissues."