As said , his shop / his terms , it you don't like it , go elsewhere simply .
When I built retaining walls , I asked for 30% + plus up-front , quite a simple way to see if people had the money or whether you were going to find out at the end of job that you would be fighting for your money . The only job that I didn't get a good deposit from , I lost 75K and my business .
With my trailer build , I put down 50% before the welder even got warmed up .
Asking him to be negotiable on his deposit terms ... good luck with one , I know what I would do if it was my business .
Ask around , surely he must have done other work , ask to see his other work and ask them what they thought of his work .