I think its great that you realize you need to protect your sobriety...not take any chances.
I can't off advice specifically. I know when I've been in the same situation I try to keep it simple. The truth is always best but if an excuse is easier than keep it simple. You already committed to 'something' (fill in the blank) else. The only reason its hard to say that is because its complicated in your head. In his it won't be so I would think he'll just accept it and move on.