Oh yeah, he is definitely noticing that the dynamics of the household are off! They are sooooo much more perceptive than they look, aren't they. He is definitely missing his daddy. It would probably be a good idea at first for your hubby to call as often as he can - ideally my opinion is that you guys should be talking every day. Of course, that depends on how much you want the phone bill to run! Even short calls are better than nothing.
Is there something of Daddy's that he can hold on to or something, like maybe one of his shirts, or his pillow? Pictures of Daddy would be nice to have.
Also, even though he can't verbalize how he is feeling, he can definitely understand if you talk about what Daddy is doing and how you all miss him, but that he will be home soon. You could get a truck (a toy one, I mean!) that looks like Daddy's, too, for him to play with, and that way things seem more concrete than "where did he go??" And talk about what he is doing.
You could get a U.S. map and track where he is going! That would be fun for the boys to see where all Daddy drives. You could put a sticker on HOME and then others on places he's heading. If you could find something where you could put pushpins/etc. and remove them that is cool, or you could just get an El Cheapo paper one and when it gets filled up just get another one! Or print something from online.
You could keep a journal, scrapbook, or memory box for Daddy to look at when he gets home from his trip, too! I am sure y'all are going through so much adjusting to his being gone, and your husband is going to be missing you and the boys terribly as well. This would help him catch up on things and not feel so "absent" and it would help you as well!
Best of luck to you!