The problem is there is no reason to work on your car. You have paid him everything. He may or may not of already bought the materials with the money you gave him. Regardless there is nothing in it for him to finish the project now. Possibly the best thing you could do if your set on rolling the dice and you think he really has the paint material is to offer him a bonus if he gets the job 100% complete in the next four weeks. Maybe $1,000.00 but don't pay him another cent till it's finished! That includes blocked, painted, cut, buffed and anything else a painter would possibly do. Your money is your leverage.
Don't let him use your car as leverage against you, if that **** starts and it sounds kind of like it has, get the car out.