As a guy, I can say that if he's doing this all the time, it's probably either subconscious or on purpose. The way you find out about it is by confronting him. If confronting him actually changes his behavior, then it was subconscious. If it doesn't then he's doing it on purpose.
He may be uncomfortable with the volume. Cutting back on how much you text him might make him treat each individual text more seriously. If this is the case then you can cut back at first and then slowly pick the pace back up. But if you make him uncomfortable again he might revert back.