Cass, I actually think that the absences have a cumulative effect on the kids. It's like all the seperation experiences build up over time. On the one hand, it means it becomes more "normal" but on the other hand, I do think it's harder for a kid to withstand the more it happens, kwim?

At this age, having lots of visible reminders of daddy can help. We had a double-sided laminated photo of daddy that DD could drag around and even 'chat' to when she wanted & you can get these daddy doll things made with a photo of your DH or even one of the service teddy bears might do the trick. It sounds a little like your DS doesn't understand the time frames involved. So I would also emphasis right from the start that "daddy will be away for a long time" and "lots of sleeps", and "mummy will tell you when it's the day when daddy is coming home." I think the calendar idea is great, I would be going one further and giving him a sticker to put on there every day. But I'm not sure how useful actually counting the sleeps is going to be until your DS gets a bit more of mental frame work re counting & numbers. For example, when DD2 was the same age, I would avoid the countdown until we got in the realm of numbers she knew and understood (ie, 1, 2, 3). As she got a little older, those numbers increased.