Hmmm this is indeed a very tricky situation! I mean you have the PROOF that you have not used yet he still doubts you!
I guess the only bit of friendly advice i can give you is to NEVER look for ' goodies ' for a friend! But you already know that now.
I will say it again, keep to your story because it is the truth and drill it home to him!
2granddaughters gave very good advice aswell, stick to the programme let him see through your ACTIONS that you are not using and in time he should believe you!
I truly hope this works out for you and congratulations on staying sober!