re: "...the car lost 2,5 quarts of oil in less than 400 miles...."
First item of business is to find the reason for the oil loss -- a leak somewhere, or are you "burning" it in the engine? Only from that point can you trace the events (causing the leak) back to the mechanic and assign the blame to him.
You cannot just say that "before you worked on it the car didn't use oil", as it can be argued that a problem (in an old car) can arise anytime and it can seem to be his fault merely by coincidence. If it was this easy, mechanics would refuse to work on any car that wasn't covered by a manufacturer's warranty.
You need evidence -- a cause for the oil loss that you can associate with the work that your mechanic performed.
Good luck.

And that being said, I have no doubt (personally) that your mechanic is responsible -- I don't believe in coincidences. But why did you use a mechanic who knew so little about Volvos (and perhaps any other car) that he didn't know about the flame trap?