Have you asked him why he feels the need to hide his friendship with these women? Because that is what he is doing and its unacceptable. If you can't have honesty, trust and communication then you have nothing
You need to stop pussy footing around the issue and sit down with him and explain what trust means to you. To me in a trusting relationship-you have nothing to hide (including my phone or the code on it and if he asked for my FB password I would give it to him)
His behavior is shady so either follow your instincts and dump him now or set some v clear boundaries and tell him you do not approve of male/female friendships. Most men are not capable of having purely platonic close female friends (unless their gay or the girl looks uglier than shrek)so tell him close female friends are a deal breaker for you-especially ones he texts in secret and hides it from you
"Don't ask a question if you can't handle the answer".