Being a business owner myself in the automotive field, I can say quoting jobs and especially a project like yours is the hardest thing we do. Legally, you may be bound to the original contracted price but remember this is our livelihood and projects working on old stuff escalates, you turn one bolt on one side of a truck and on the opposite end a power steering line starts leaking, it’s just how it goes. However, he should have informed you that the project has grown and he’s going to need to take more time than originally estimated to do a proper job for you. It’s not a good way to handle the situation for you to just hear about it at the completion of the project, I’m not sure from reading this if that is the case or not, but either way you should meet with him, go over the contracted work and have him explain how it grew from there and what extra work had to be done, and find a common ground. You never want to burn a bridge with someone who’s worked on a project like that for you if you can, because more than likely someday the truck may need some more work, and he will know it better than anyone.