Pregnant, Boyfriend in Iraq, and how do I recognize symptoms of post traumatic stress disorder?

I am 7 months pregnant and my boyfriend has been on his second tour of Iraq since may. He'll be home in January for the birth of his child and then gone again until at least August. He is usually very attentive, loving, and a wonderful boyfriend...but lately I've been noticing a change. It feels like he's distant...maybe depressed who knows. I'm afraid of what he's going through and seeing over there. I don't know what to do or how to deal with this. I want to hold on & be strong for him but I'm also afraid of getting hurt so I've began distancing myself emotionally as a way to be prepared incase. We're having a baby and I'd love for this to work...he's been a blessing. Maybe its just getting comfortable in the relationship but I don't know so perhaps I just need some advice or guidance. Anyone else ever been through this? And can you tell me what initial symptoms of P.T.S.D I should look for or how you got through it? Any advice is appreciated and thanks to all that answer in advance