I remember when my young daughter had surgery and did not need the pain medicine after a day or two. I just put it in the cabinet not thinking. Sure enough, my X drank it. Then when I had surgery he was slowly stealing my pain pills as well. He tried to deny it, and he tried to make me feel crazy, but in the end admitted it b/c he knew I was not buying it.
Your heart wants you to be wrong about it. Your head knows what he is up to. Now it's up to you to decide just how much you are willing to put up with.
I know it's hard. However, this is his issue you are trying to control for him. That won't work forever. I imagine you feel very lonely, or at least I did when I was in your position. Just opening up and telling the truth to those around me helped, a lot.
Boundaries is a wonderful book. I went to a seminar given by Dr. Cloud as well a few years ago, it was amazing.