Answer: Marines
Interestingly enough, I thought he'd be in the Air Force. 1: Steven says this to Jimmy when he tells him to put his mask back on in plane. 2: Letter to Wil Smith CLEARLY reads USMC, El Toro United States Marine Corp Base. 3: Jimmy even reads the letter to Captian Hillard saying and I quote, "Captain Steven Hillard..loser. United States Marine Corp...we regret...."