Can I get some input from a mechanic's view point?

I'm having a conflict with my mechanic. I've used him for about 5 years for my various cars. He is very good, and reasonably priced (but not a super bargin, just fair). He is also very honest, which is why I keep returning. I've probably spent around $20,000 at his shop over the last 5 years, so I'm not just talking a few oil changes.
He took in one of my classic cars for a front end rebuild - about 15 hours shop time and $500 in parts. Problem is, he has had my car for 3 months, and it is not finished. I call him every other week and ask, and he apologizes but says he is "just so busy." This is true, he is busy.
At what point am I entitled to some sort of compensation for him keeping the car for so long? I pay $150/month for insurance for a car I can't use. A 15 hour job with common parts should not take three months. I'm not expecting a 2 day turn around and I do have other cars to drive, but this bothers me. What is a mechanic's input on this?