First, stay calm, yet firm. Clearly state the reason that you are not satisfied with the services and what you expect the contractor to do to fix the situation.
Be calm and explain why you are not going to pay him. Document with receipts and pictures why not. Politely explain the small claims process to him anyway.
You should act in a professional manner that is becomeing of you and your contractor, assuming there is reason for nonpayment, and explain ytoyour nonpayment in writting.