This may sound like a broken record to many that frequent this site, but your best bet is to order his IPDF ( individual deceased personnell file ) from the government . There are instructions at message # 2112 on this site . Do you have his service number ? If not I can provide it to you.
The IPDF will have an internment ( Burial ) report which will show cause of death. Many times it will say " Shrapnel " " GSW ( gun shot wound ) " ETC.The report was filled out at the time he was initially interred overseas, ussually a very short time after the date of death.