This is what I would do to. Prove it! He can't do that without witnesses imo. I get grass on cars a lot when they are parked close to where you are doing trim work. Sometimes the wind throws grass on dew laden autos when you are blowing off drives. Grass on a vehicle does not prove you did other things to it. I would take my chances in court. This is the problem with Insurance. You make a small claim, then you will pay a big price in the long run when they jack your premiums up. Time for your annual audit!