man, I can kinda see why you'd be confused. I hate this too. here's a thought, why don't you try giving him a call sometime instead of just a text? do you talk to him in any other way other than texting? have you met him or hung out with him? I'm only asking because if you're trying to figure out if he's interested in you, it's not gonna happen through texts.
so you've already noticed the times he doesn't reply and the times he does reply, now what you could do is only text him when you know he's going to answer, that way you won't have to worry when he doesnt, see? and who knows maybe you can text him in the evening and casually ask him "so what do you do all day?" or "what do you do during break?"
and you say he never texts first, I feel your pain! lol when a guy pursues a girl it most likely means he's interested, but when the girl is constantly pursuing the guy he might have the tendency to run for the hills. I don't know they like a little chase, and I gotta say we girls do too (just not as much as them, it gets annoying! hah)
so let him know you do like talking to him but also let him know that you can do just fine if he doesn't text you. wait for him to text you even if it's a couple days, that way he'll be like "hey, she hasn't text me lately, wonder what's she's up to" =o