A man who has a sex partner back home will want to be plunging to hit bottom as soon as the aircraft hits the runway. Most likely other servicemembers have been hazing him for being a father to a child whois not his. He is breaking under pressure. If you want to keep this man, communication is the key. The bonds and influence which his fellow servicemembers have is huge.
Be blunt. It is either a win or lose situation. I had this psycho crap pulled on me in the first gulf war. How did I know if my pregnant wife's baby was mine? That type of talk is what he has been enduring, but the cruelty could be greater. His buddies put shit in his head. you have to fix his head.
Avoid an arguement at all costs. Discuss it in a public setting, but not so evryone can hear what is going on. Let him know you are there for him, and you have been faithful and loyal to him. Guys love these words.
Gently prod him for answers, he will open up and this will be a win or a lose for you. It's the only way to know. Not knowing is the worst. Best wishes!