|Sorry about your brother's problems, I can relate, many friends with problems.|
My cousin was in the Air Force, and may have been over there about the same time. He was stationed at Tan Son Nhut, and had the worst job you can imagine. Graves registration! He was responsible for "bagging and tagging" every soldier that died over there. I think about 1964 or 1965.
And, I guess I just assumed your brother was Army, but MOS is your Military Occupational Specialty (your job). 11B was a grunt (not derogatory), infantry. I was 13A (artillery), we had the 11b's backs.