Well...if you REALLY wait til he's ready, it could be another 5.5 weeks! ;) Anyhow, I think one of two things could be going on. Do you know when you ovulated? If not, then your due date could be off by up to 2 weeks. Using your LMP or an ultrasound to determine the guesstimate of "readiness" isn't as accurate as knowing when the pregnancy really started. So, you could be farther along than 36.5 weeks. (FTR you could also be as little as 34.5 weeks, the guess can be off by up to 2 weeks in either way).
Otherwise, it could just be that it's your second. Many women feel their bodies ready in preparation for labor just sooner with subsequent pregnancies. The body just remembers what it was like to open up and give birth and it usually happens sooner than the first.
Don't pay much attention to their guess of his weight. They should have told you that it can be wrong up to TWO POUNDS!!! The later in pregnancy, the more off they usually are:( The fundal measurement and NST are a good idea if you have bad GD, but the weekly U/S don't prove anything and there is a lot of evidence that they aren't nearly as safe as OBs thought they were (kinda like the x-rays every pregnant woman used to get until they realized they weren't safe after all).
Right now there's no medical reason for an early induction (and no, "Big Baby" doesn't count). "Fully Term" doesn't mean his lungs are ready and plenty of 39 week old babies have been induced only to find out their lungs weren't ready. You're just asking for a stay in the NICU when you induce early for no reason. Hang in there! Take warm baths, go swimming if you can, and check into getting a maternity belt that will take some of the pressure off your pelvis and back. There are a few different types made by a bunch of companies.
http://www.google.com/products?q=pregnancy+belt&oe=ut...
Good Luck!!! Hang in there, no one is pregnant forever :)