This whole thing stinks. You are a nice guy that is getting shafted. If your sister is such good friends with him why doesn't SHE get on his ass over getting your truck done? As I wrote the previous sentence, I knew that would never happen. If he is going camping with your family, can you show up at the campsite? And in a very nice manner, tell him that if he hasn't done anything to your truck, it's ok. You'll just arrange to have it picked up and you'll take it somewhere else. If he hems and haws around, screw him--tell him that you are going to sic a lawyer on him. And with a sweet smile, tell him that you have plenty of money to throw at a lawyer and you are quite happy to make it cost him a LOT of money. Your next sentence needs to be that you are going to pick up your truck. Period. Or get ready to do legal battle and spend more money than it is worth to him. Let him know that you have a client that will handle this for you as a favor and you don't mind one bit dragging him into court and making him spend lots of money on lawyer fees. Be nice, be firm and don't take any crap off this yo-yo.
