I've had that situation as well, but I was in your boyfriend's position. I have one partner that I live with and another who I'm in a LDR with. I was traveling with JJ (the one I live with) for 3 weeks and I couldn't be in conctact with rory much during that time. It was actually quite stressfull for me, because I really would've wanted to contact her more during that time but it just wasn't possible. Usually we skype each other at least every other day and talk several hours. So this trip was a really big change in that. During the whole time I talked to her maybe like 2 hours. We wrote more messages though, which was easier because we were on different time zones as well. I took the time to write the messages before I was going to bed or early in the morning or whenever I had the opportunity.
I'd recommend the same thing as Annabel, make sure he knows how important it would be for you that he makes the extra effort to contact you when he can. But you also need to accept that it's going to be different than normal. But it's not for forever! When he returns to his normal routine, you'll get to speak to him more again. Hang in there!
Oh, and I think you might get something out of my and rory's blog here on the lifestories and blogs section. I've been in a LDR with her for almost 8 months now.