I am not sure a contract between the two of you can be managed without a mediator (a counselor who acts as mediator).
Often in marriage counseling, when one partner has cheated on the other, these kinds of "contracts" are made, in which the "cheating" spouse promises henceforth to be transparent with the partner. The "cheater" communicates frequently throughout the day and night while away and reports consistently where he/she is, what he/she is doing, when he/she will be home, etc.
In your situation, without a professional mediator, I'm not sure a contract will work, as it does set you up to police him, in a fashion, which is unfair to you and can stoke his rebellion and resentment if he feels he is being parented.
I think your gut, which you mentioned early on, is your best ally and I have found in my life that when I need to know something, it will be revealed.
Do you work a program? Maybe some work with the first 3 Steps would help you very much right now. Bring some peace.