not trying to be a jerk, but there's an old saying that i always see on signs at car repair shops:
WE DO WORK 2 OF 3 WAYS: QUICK - CHEAP - GOOD.
- if it's QUICK and CHEAP, it won't be good.
- if it's GOOD and QUICK, it won't be cheap.
- if it's CHEAP and GOOD, it won't be quick.
really, though - it does suck that he's taking so long. just remember, you catch more flies with honey, and being nice is going to motivate the guy to work on it better than if you piss him off. also, i've found that randomly dropping by to view the progress is a great motivator for people.

good luck! i'm sure you'll be on the road quicker than you think.
EDIT: i see that this post was redundant -- apologies! but it *is* true...