As long as he's still in a relationship with the other girl, you should stay away from him physically and emotionally because if you don't ~ then more likely then not, you will be setting yourself up to be that "side piece" you so don't want to be.
He has a girlfriend right now and for all you know, they may have set it up that they do not have to be sexually exclusive while the two of them are apart. Believe it or not, many couples in the armed services have this arrangement. You don't want to be his sexual playmate until she comes back from where she's stationed.
You tell him that its inappropriate for you to be seeing him off when he's in a relationship with someone else and then you wish him well and to be safe ~ over the phone.
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion