2002 Isuzu Trooper repair

My husband has a 2002 Trooper that had crankshaft problems. The mechanic we chose told us to give him 2-4 weeks to make the repairs and an estimate of $3500. We gave him a deposit of $1500 with the remainder to be paid at the time of pickup. That was December 10, 2010, and our vehicle is STILL not repaired. He continues to make excuses as to why it's not finished. We've heard everything from, "We're waiting for the crankshaft to be repaired" and "Isuzu parts are really hard to find" to "It's been apart so long we're having problems getting it back together." He's told us numerous (6-7) times that it will be finished on a certain day, with no results. He told me on Monday (3-28-11) it would be done Wednesday (3-30-11), but again, we got no call from him. I feel like this guy is taking advantage of us. I know nothing about car repairs or how long things should take. Is four months a ridiculous amount of time to fix a crankshaft? Are the parts really that hard to find that it would take 3 months to get them (that's what he told us)? Can he hold my vehicle if I refuse to pay him due to his unprofessionalism? I just don't know what my next move should be with this guy. Does anybody have any ideas?