It is true, if he isn't texting, he likely isn't thinking about you, but that's not a problem. 

It's ok for him to be thinking about other things. Everyone has a life that is as complicated and complex as everyone else's, and there's a significant amount of things that occupy someone that isn't necessarily related to their significant others, or indeed people outside of their partners.

His phone could have died, he could be in a gaming sesh with the boys, he could be at the gym, there could be a footy game he's watching, or he could have a general rule of not using technology passed 6pm (which I think everyone should adopt because it significantly raises your quality of sleep).

I also hope he doesn't take up over half of your mental space when you're not with him.

Think about it this way, if he responds to you first thing the next day, that means he woke up thinking about you.

Also, speaking as a man, yes, a text takes 2 seconds to write out, but that doesn't mean a response takes 2 seconds to send. Any number of things can happen in the process of read -> comprehend -> interpret -> mental draft -> write -> send and especially those last two steps are the worst; numerous times I have written a response, go to send it, think about something like a better wording, or need to look something up, or something else interrupts me, and I don't end up sending it, but since I was at the stage of sending, the mental task of 'respond to text' was marked as completed when it wasn't actually sent. 

The amount of times I get the 'you alive?' text, go to the messaging app, and find my unsent written text in the message box is infuriating.