It depends on what he wants more -- to be in a relationship with you, or to sleep around with other girls.
I was dating a guy who joined the military a few years into our relationship. Joining the military affected him a lot of positive ways, for example, he became much more confident and had increased self-esteem. But he also started partying a lot with his military friends and many of them were out to pick up and sleep with girls. I was his first girlfriend and the only girl he had had sex with, and I know he felt jealous that his friends got to "have all the fun", but he had to be good because he had a girlfriend at home. I know he didn't cheat on me, but at one point, he asked me if it would be okay for him to try asking girls for their numbers (he didn't want to call them, he just wanted to prove to himself that he could). I could definitely see hanging out with people who party and sleep with random girls a lot having an affect on how he feels. People want to "fit in", have fun, and do the things that their friends are doing. That being said, it's really an individual thing when it comes to how much this affected him and whether or not he felt the need to engage in that kind of activity.