My husband travelled every week, when the kids were young. He was usually gone one or two nights - every now and then, it was three nights in a row. Three nights could be brutal!
My husband called EVERY NIGHT! He always asks about our day and we ask him about his day. It's funny to hear my kids say things like... "So, how's that long ride with Jim? Did you make the sale??? What did you have for dinner?" When they were younger, sometimes if it was a lengthy trip, he would call twice - once in the daytime. And we end the call with, "I love you. Have a safe trip." He has always brought home SOMETHING. Whether it was just a bag of peanuts or raisins from his flight, or crackers, anything. If this is ongoing travel, just be consistent and watch your own body language. Make sure you are positive and consistent when your husband leaves. Change any wording of .... we're going to miss you".. to... "when you get home, on Thursday, we are going to (i.e. have a nice dinner, play a game, etc....) Just try to stay upbeat. Every now and then, have him make specific phone calls to his son, too. Your son should ease into this travel. There may still be nights that will be difficult but just try to be matter-of-fact about the travel then try to distract your son and get him on another subject.
Then, when Dad walks in - make a big deal! It's cute even now, when my husband comes home, my son is 11 and he carries Dad's briefcase to his room, then begins to dig through it, looking for any cookies or treats that were stuffed in there. And, I don't know why this was, but, when my son was younger, it was always harder on my son, when my husband travelled. My daughter was much more matter-of-fact.
Good luck to you... yes, it can be exhausting for you! Just don't ever let Dad "sneak out". I think that's much harder on a child.