A.,
First let me say that I am so sorry to hear about the loss of your husband and your son's father in Iraq. My husband is in the USAF (20+ years now) and I cannot imagine what it has been like for you. I think it is entirely normal for your son to be acting this way. While he is young, children are exremely smart and he surely knows something is not right. People talk and don't think about him being near or in the room; and he can most likely "feel" your moods, feelings, tension, sadness, and other emotions that go with your loss.
If I were you (I am a very well educated, M.Ed. HS English teacher who is currently a SAHM w/ a 4 year old little girl) I would take him to bed with me every night that I possibly could. I know the trend is for kids to sleep in their own bed but your situation isn't normal and if he needs you for reassurance, so be it. My daughter sleeps with us most nights. My parents were huge advocates of the family bed and all three of us sisters would sleep with our parents when we were younger. All of us are well adjusted women with children of our own in healthy relationships.
I love waking up with my daughter and seeing her first thing in the morning. Two years ago she almost died while we were living in England. So, I cling to the fact that she is still alive and is now healthy. Take him to bed with you, hold him, cuddle him, spoil him...and in return you are comforted, held, and secure feeling.
If you ever need anything please email: ____@____.com. I think you are extremely courageous and wish you the best of luck with your endeavors and with raising your little boy.
Again, my condolences to you and your son, and the rest of your family.