Wow, he sounds just like my horse. Oliver is go-go-go but as soon as my butt hits the saddle there's nothing there. I've had him checked by several vets and professionals and they all say the problem is between his ears.

I haven't read through everything - so someone may have already suggested this:
The trick though with a lazy horse, is to not nag - that just pisses them off (Oliver bucks when I use spurs/whip as well). My instructor told me last weekend: riding is a partnership with your horse, but it's not an equal partnership, and right now he's pretty sure he's in charge.
You have to give a really big clear aid (I really whollop Oliver, and then hold on) and as soon as you get the correct response, stop. Be quiet, that is the reward. When he starts slowing down - give the aid again, make it very clear. Then stop.
For me, I tend to say "go" with my seat, but then I hold to much with my hands, so the trainer makes me put my whip in one hand and both my reins in the other - it apparently helps "open the front door" so I whack him with the whip and then just sit there, it's worked great at both the trot and canter...though it gets a little scary when he really gets going fast