Hmm perhaps he doesn't have such a high opinion of your bright and witty texts as you do!
Seriously, I would guess the reason is that he isn't interested in having his day disturbed 50 times at five-minute intervals by taking part in a text conversation. It's not everyone's idea of fun. So he acknowledges you to keep you happy, but is careful not to do anything that might make you send him another one straight back.
Why does he reply some days and not others? Well, I expect that if he was given the choice he would reply to texts only when he has absolutely nothing better to do. Some people have to be really, really bored before a text conversation starts to seem like a fun way to kill half an hour. On the other hand, he's no doubt aware that you enjoy this sort of thing even though he doesn't, and so he tries to make the occasional effort to reply.
In short, you are over-analysing this. Don't sweat it. There is no possible way that you can tell whether a man wants to shag you based on how long it takes him to reply to a text, how many words he types etc. Best way to find that out is to ask him if he fancies a drink sometime, just the two of you, and see where it goes from there. Brightness and wit generally come across much better in person anyway.
Just don't ask him out by text because then you'll only spend the next six hours trying to work out what he "might have meant" by his answer!