Some people just don't feel the need to be in constant communication - it doesn't mean they don't care about the other person but they prefer to focus on things which are actually present in their life and won't text if they don't have something to say. Both myself and my partner don't talk a whole lot if we're on a trip or visit somewhere, maybe a text in the evening or something but occasionally if we're busy we go a couple of days without talking. I think you need to understand your boyfriend's perspective too and that he's not doing something cruel or lazy, he's just got a different view to you. Plus it sounds like he's working really hard - in which case you should cut him some slack. Ideally you would compromise and maybe he could agree to text you a couple of times a day and you would agree that if he;s busy you won't moan if he forgets sometimes or can't skype. Perhaps you should have a conversation about it and explain how you feel and request you both compromise.
... and the ones that won't