Military mom here...
I actually think that less communication is better, if it's done well. My children seem to find their groove, like I do, and when dh calls it throws us all off. The more matter of fact I am about how our life is, the better they do. Our kids understand WHY Daddy goes, and HOW it benefits them. So, it's just a face of life to them.
Sometimes they are sad and scared and angry, but mostly, it just "is what it is", so we do okay. Ds (who is 2) would not let me sing certain songs or talk about certain things with him, though, this last deployment. It was obviously Daddy's turf, and it mad ds very sad to go anywhere near there.
I talk a lot about dh, about where he's sleeping, and eating, and as much as I can about what he's doing. I talk about how much he misses them and how much he can't wait to be home.
We also try to always have a carrot dangling out in front of us: "When Daddy gets home x will happen." Also, "When Daddy goes to x, we are going to y." It really helps get the focus off that major event of him leaving, and then excites them about when he gets home.
By 6, though, I think your ds needs a lot of honesty. You didn't mention what your dh does, but I would explain as much of it as you can to him. It has seemed to help dd (in lots of things, not just her daddy being in and out) to be told, "you are probably going to feel x, that's okay. We can talk about it if you want." And, "Mama is sad too when Daddy is gone, but I know that y, so it's okay." Also, "Did you know that when people love each other and one of them has to go away for a while, they still love each other? Did you know that when people are separated, they feel certain ways. You are going to feel this way, and then this way, and then you'll see, etc..." (google "emotional stages of deployment".) Just knowing what is going to happen can help a lot.
You said you often don't know when he'll be back. That happens to us, too. Or, if he'll leave again right after he got back. We were holding our breath that he'd have to go for a month or so right after he got back from deployment. Looks like we're in the clear, but...
Anyway, don't dwell on that aspect. Instead of talking about WHEN he's getting back, talk about WHAT you'll do UNTIL he gets back. Without pushing your dh out of the picture, talk up the benefits of daddy being gone. Make it fun. DD knows that if Daddy had most "regular" jobs, she wouldn't get to travel to see family all over the place, and that Daddy wouldn't get a month off sometimes.
I'm feeling very random, but I'm in a hurry, and my mind is fuzzy because it's late. Bear with me, one more suggestion. When your dh talks to the kids, we've found it very helpful to have something specific to talk about. When you are just trying to catch up on the phone and talk about your day, the conversation falls flat, and the relationship suffers. Real relationships are built on shared experiences, which are hard to come by when you are separated. Maybe your dh could start a coin collection with your ds, and every now and then have one to tell him about. Maybe they'd like to build a tree house, and your dh has this incredible idea to expand the left side. What does ds think? Maybe, they are reading a series of books, and dh found the next one. He'll read it when he gets home, but listen to what the back says! Stuff to make sure your ds still feels connected in a real way to his daddy.
Okay, two more things. I had a large group of friends here for a while, until everybody but us moved, and we all went through a deployment together. And a bunch of us had kids the same age. For ALL OF US, every single one, we had a very rough first 6 weeks. It was textbook. FWIW...that's just how it goes. I will say, though, this last deployment was much better. I planned a lot of distraction, and I think that helped. It was still rough sometimes.
The last thing is a story, to illustrate how kids sometimes perceive things differently and why they need a lot of blunt honesty...
A friend had a three year old who watched his daddy get on the bus and leave. They had had LOTS of conversations about everything, and continued to have them. Months later, somebody asked him about his daddy. He said, "I don't have a daddy." His mom was SHOCKED. When they asked him why, he said, "He got on a bus and went away. I don't have a daddy."
Even if you think he knows, even if you think he understands, some fishing may be in order to be certain he knows what's going on.
I know that was long and all over the place. I hope there was something in there that could help.
