He’s swamped with work.
He’s worried he doesn’t have anything interesting to say.
He’s out having fun with his friends.
He is trying to play hard to get.
He’s out with another girl.
He’s writing and rewriting his text, trying to come up with something clever to say.
He doesn’t want to lead you on.
He hasn’t actually checked his phone in hours.
He’s not in the mood to talk right now.
He’s not as interested in you as you are in him.
He doesn’t want to come on too strong.
He’s busy texting other people.