to get benefits you will have to file a disability compensation claim and present your supporting evidence
It might have been one of the things they were in contact with. If they served in the Republic of Vietnam or in or near the Korean Demilitarized Zone during the Vietnam Era or in certain related jobs they may have had contact with Agent Orange, an herbicide used to clear plants and trees during the war.
May I ask something regarding military disability compensation, he also thinks he got sick because of asbestos besides the weapons, so is that possible?
Yes, there is a possibility he may have had contact with asbestos while working in certain military jobs. Asbestos toxic fibers were once used in many buildings and products.