How can I approach him about how I feel? And to know if he is hiding more?

I’m 20 years old and he’s my first and official boyfriend. However, being with him for almost 5 months is great but, to know about what he told me what he did with this close friend of his, I feel as if he isn’t telling me everything. When his friend moved back into the state, he seemed okay to hangout iwht her…I wasn’t expecting it for him to be “alone” with her either. Taking her places and regretfully taking her to a nail salon, then to his place. It made me utterly uncomfortable and I never heard from him all day either. After that I approached him on how I felt, but what proof he gave me that he didn’t do anything doesn’t make sense…and this was about a week or two ago as well. I don’t want to go back on that, but I still have thoughts about what else he may have done, but I’m scared to ask and tell him that he might be hiding things, because that’s evading his trust and he may think I don’t trust him. Being scared to this point, I hate complaining to him about my feelings, I feel as if complicated to him to make him hear these repeated feelings over and over. He always tells me that I can come to him and tell him what I feel, no matter how many times I do…but I just need tips or hints on what I should do if I feel he is hiding more than I know…please and hank you..