[Shuie]

Can you ship him the unfinished materials and let him do his work and then ship his finished part back in batches, or does he need access to your shop and tools to do the work?

I live by the 'everyone is replaceable theory' when Im at work because everyone in my line of work really is replaceable. From what I understand about your line of work, EVERY detail and technique can make a huge difference in the performance of the finished product. I know for a fact that you have demanding customers that obsess over small details and nuances. Unless you are willing to replace this guy yourself and are certain you can train someone to do exactly what he does, then I would continue to work with him.