I suppose I agree with Evelyn, but I do find it quite strange that he won't even tell you where he is going to be. Has he entered a spy program? Are the cops after him for any reason?
The only other thing that I can think of from my own experience (not with a girlfriend, but with my parents) is that I have from time-to-time been a little vague about where I am going out of concern for freaking them out.
By example, this past fall I made all of my plans for visiting Myanmar and had basically just told my parents I was going to Southeast Asia. All along, they assumed I was going to Thailand, Vietnam, etc. When I finally told them I was headed to Myanmar about 2 weeks prior to departure, they totally stunned me by being psyched about it.
Not sure if your boyfriend is doing the same thing, but maybe?
(from travel advisor to relationship counsellor, here we go!) Sorry to be so blaze, but if it's meant to be it'll be. Worrying about it and obsessing about it (we've all been guilty, don't worry) won't give you any answers, it'll just make the time now until he goes uncomfortable and while he's away even more horrid. See it as time to do things for yourself, do something new. You never know, it may be the last taste of time alone you get , maybe you'll get a big break or, shock-horror meet someone else . I guess what I'm trying to say is, there is nothing you can do to make it work or not work, you are in a catch-22 where the only reasonable choice you have is to put a smile on your face and go with the flow. Sounds to me like he wants to exert his independence - and if that's something he needs to do, then it's healthier to let him get on with it. Whether you're still there when he's done what he needs to do is up to you.