Quote: I know that I shouldn't question him on his whereabouts but in light of the sitch it is because I don't trust him.
1) Why shouldn't you ask him where he has been? He betrayed you, trust takes time to rebuild, it is perfectly reasonable to want to know where he has been. H is committed to rebuilding your M, which means rebuilding trust. I suggest you reread the chapter on infidelity in DR.
2) I expect it isn't that you ask where he has been, but HOW you ask that counts as mothering. You tend to try to catch him in the act, you quiz him, interrogate him. (Then, even more annoying--sorry, it is true -- after you get the info you were after you backpeddle and act like you didn't even care or need to know...) Anyway, try to treat him like an adult. This means that YOU need to act like an adult and do the hard stuff, like genuine communication. Trust me, I know it is much easier to catch and quiz than to engage in adult interaction. Adult interaction would look something like this: "H, I saw the cell phone bill. I feel betrayed and stupid because I found that you are still talking to OW. I would like you to tell me about the calls and I would like to know your intentions with respect to future communication with OW." Then, give him a chance to talk.
Direct communication = direct reports about yourself and direct questions to him
And, you need to figure out your boundaries and communicate them. Right now, you are talking out both sides of your mouth -- on the one hand you tell him it is fine if he talks to OW, on the other hand I'm sure that you are also letting him know that there will be hell to pay if you "catch" him again.
You are afraid to set a boundary, as a result, you are sabotaging your M.
There is NOTHING unreasonable about wanting to know where he is and wanting him not to communicate with OW.