More than likely this doesn't reflect anything other than the fact he's absent minded. I've been known to do this as well. I don't mean any offense, and I'm not trying to be impolite. I just get focused on going about my daily routine, work, or I get really into a video game, and stuff just slips my mind. It's just the way my brain works. I would not conclude that he's angry with you.
Remember, if things like this bother you, you've got to talk about it. He will most likely be utterly clueless that this was even an issue, because he only has himself as a frame of reference, and he probably isn't in the least bothered when people respond to his emails a week later or never. When someone doesn't respond to my e-mail, I usually conclude they were either too busy to read it, or had nothing to say, or w/e. It would never occur to me that someone was angry with me.
The important thing is that you talk to him about it. Just say that you like talking to him and you miss hearing from him. If he knows it's important to you, it will make a difference. Have you considered setting aside some specific time each day to talk, like with text messaging over computer, or telephone, or w/e? Working yourself into his routine might be a better strategy than trying to get him to interrupt it.