Before my husband and I went away on our first extended vacation, I talked to my little sons about where we were going, showed them on a map where we lived, where Daddy and I would be, how we were going to get there and back, the kinds of things we were going to do, marked the dates we were leaving and coming home on a calendar (even though I'm not sure how much they understood the calendar). We talked about how many days I'd be gone, counted them on the calendar, etc. Then I would throw things into normal conversation like, as I was fixing breakfast, "When Dad and I are away, Nanna will make you breakfast in the morning," or, "Next week, Nanna will take you to your class." I think that helped them realize that much of their routine would be the same but there would just be someone else helping them. I also talked about how they were going on their own "vacation" to their grandparents' house.
Just be very matter-of-fact when you talk about it, but don't brush off his concern about you leaving. Let him know you understand that he might miss you and be a little sad that you're away. Reassure him that you'll come back when your trip is over, and give him something he can pull out and think of you if he needs to while you're away. It could be as simple as a picture of you or a heart he sees you cut for him out of paper. You could also ask him if he could give you something to take on the trip--maybe a picture of himself that he picks out or a picture he draws for you.
The other thing I was torn about was calling to talk to them. I was more than happy to check in every day, but most of the time I just talked to my mom and not the kids. I never called before bed time since I found that calling then can be more upsetting than soothing, and when I called during the day, they were often too busy having fun to want to stop and talk to me. Although I missed talking to them, I was happy that they were so happy.
And just remember that even if your son has a rough start to the week, he'll probably be fine after a day or two. It's all part of growing up. Good luck!