In the first episode we see him as a JAG lawyer in the rank of lieutenant, and follow his career from that point as the series progresses. Neverthless, some background information on his past life and military service is given, as presented below.
Rabb was born on October 25 1963 in La Jolla, California (cross-check date and place) to naval aviator Harmon Rabb, Sr. and Patricia Rabb (maiden name: Reed). His father was considered MIA after his plane crashed in the Vietnam War. When he was 16, Rabb went to Laos to look for him together with colonel Francis Stryker.
Rabb served as a pilot for some time (how long? on what ship?). After an accident during a night landing on an aircraft carrier, in which his RIO dies, it is revieled that Rabb sufferes from night blindness. He is transferred to JAG after this incident.
Promoted to lieutenant commander in episode 1x13 "Defensive Action".
Promoted to commander in episode 0x00 "Title".