I haven't heard from him all day?

I am getting to know a guy long distance, we met online. I am really struggling to figure him out. In the next couple of weeks I plan on visiting him for a day (he lives in another city, short plane ride away), he seems excited. I have noticed sometimes it takes ages for him to reply (12hrs or so) or another text from me. We Skype chat a couple times a week and we spend hours chatting. The thing is I cannot figure out his text habits, one moment seems disinterested, next moment texting me good morning/night 1st. So far the most texts I have sent unanswered, hours apart was 4 and 2 hours, no response. He usually texts back as though nothing happened. What is going on?

Is he a very busy person in school or at work? He might not see the point in messaging as much anymore until you both meet up in person and see if things hit off between you.
If his texting habits bother you that much just tell him, be like what's your deal with being super keen texter one minute and barely realising I'm there the next?
He's probably just not a big texter though

I'm wondering if it is that because from experience if a guy wasn't interested, he wouldn't reply at all. Also if I'm not interested in a guy, I just stop replying completely.

I'm the same, if I don't find a conversation interesting or aren't into him I just forget to reply or don't bother replying. He has agreed to meet up with you which is a sign he is interested in you, otherwise he would've just not bothered. Interesting though that you're visiting him as opposed to him visiting you

I actually initiated meeting up, I want to meet him as soon as I can instead of sitting here wondering what he is really like. He did casually mention visiting me, but I want to go soon just to feel him out.

I think it will really help you to meet him, you'll see the real him rather than the idea of him that you get when you only see the online version of someone. Best of luck and drop me a PM if you need any advice with this guy

I'm actually kinda freaking out over his lack of reply, sometimes I feel like I annoy him, but he did tell me he was insecure and needed a lot of reassurance in a relationship.

What was the last thing you messaged him? Was it something he could reply back to and continue the conversation easily with? I'll say until you meet up in real life and gauge how he is in person, keep talking to him (even if it feels unnatural or forced or like you're annoying him because that feeling might be in your head and he might not see it like that)