Potty Training Woes

I'm soliciting advice. I didn't think I'd need to. I thought I knew how to potty train, but apparently I don't. Jacob was completely potty trained at 2 1/2 and it was only a two week process. It must have been all him and nothing to do with Mel and I though, because Kyle is a completely different story.

Here are the facts:
*Kyle is not afraid of the potty. He will willingly use the potty if we take him in the bathroom and help him.
*He has pooped on the potty, but it was a long time ago. I haven't been able to get him to sit long enough to do it again.
*He tells us if he is wet or poopy and wants to be changed.

The pitfalls:
*If he is wearing underwear, he will poop or pee in them without a second thought and then demand to be changed.
*If he is naked, he will pee wherever. I even discovered him peeing behind the couch once when he didn't have a diaper on. Sneaky kid was hiding like a little puppy.
*He has never initiated needing to go to the potty. He will only use the potty if we tell him to. We even went two weeks where I took him to the potty every 1 1/2-2 hours thinking he would finally "catch on" and get in the habit of going, but to no avail. If I accidently forgot to take him and he had to go...he just went in his underwear.

So my million dollar question is this - At what point should he be saying "I have to go potty?"
Oh, and I've read a couple books and articles on potty training, but I've never seen this particular problem addressed. Maybe no one else has this issue... or a child so stubborn?