Akward silence

I've been with my boyfriend for four years and I absolutely adore him but we live on opposite sides of the country so we can only talk by texting or calling. My boyfriend has a super hard time with social interactions and one of the things he cant stand is silence, he gets very uncomfortable and just hates it deeply which is really difficult when we call. I love calling him, hearing his voice and just having his attention makes me very happy so I'm perfectly fine with sitting in silence, hell I'll smile the entire time just because hes keeping me company but he cant bare it. He'll fidget and make noises and get agitated when we have a bout of silence which is bound to happen because we call for hours on end and for two people who are not great at conversations the silence lasts a while before we find another topic. He makes me so so so happy and I want to be able to return the favor as best U can, so how do I make it easier on him? How do I make him not feel so "akward" as he puts it? I try my best to keep the conversations going but at one point or another it always falters and I feel awful for making him feel the way he does when its quite. So what can I do?