Most Helpful Girl
Yeah... either he's a super private/paranoid person and doesn't want you to see his conversations even if they're totally harmless and innocent... OR, he's texting another girl. And here comes the tricky part... he might hide his phone because he doesn't want you to suspect anything and become jealous even though (!!) his texts with this girl are innocent. He's scared you might react badly. Or, the obvious (and probably most plausible reason) his texts with this girl aren't so innocent and he doesn't want you to find out that he's cheating.
Regardless, you should probably ask him why he's being so secretive about his phone.