I just had another thought. Draw up a contract between you and he guaranteeing him the job at an agreed upon price. Once the material is on site, I would include a start (window) IE: 1-2 weeks? and estimated finish time. His 5% upfront fee is likely there to keep you from changing your mind and him getting stuck with the materials which is to a degree understandable.
With the above contract notarized etc. YOU then buy the materials at his cost and have them delivered directly to your property. He still get's his profit/ markup and you at least have the materials if somethings happens to him.