relationship help!! secrets?

IMHO, such clearly secretive behavior could be something to be concerned with. Open up a non-accusatory discussion about how you feel with this behavior you're observing...(hiding who he texts). It treads over the line of acting untrustworthy and allowing you to feel weary of what his motivations are.

See where that discussion goes. If the talk goes quickly towards defensiveness, consider telling him you're considering backing off on the relationship due to the need to protect yourself.

Whatever you do, though, NEVER snoop into his phone log. That would be a behavior that might be the 'cause for HIM to feel that you're the untrustworthy one.

Yes of course. You have to tell him that you just want to be cleared. Because relationship is stands on belief. Such type of behavior can harm your belief. So you need to calm, and need to ask him politely and you have to find it gently.

Absolutely

If he was meeting her in person and talking with her this often, would you be concerned?

He owes you an explanation.