Dear H. P,
When my son was about the same age, I had similar problems. Fortunately I lived out in the country and was able to let him run around naked so he had a better understanding of his bodily functions; I taught him to "pee on the tree!", etc.
It seems to me that the "dirty pants" don't bother him because he's done it all his short life. It is just a natural fact that he knows already.
I don't know if it's inherant to boys, but I also had problems with him going poop on the potty, so I had a bunch of empty Betty Crocker Butter containers (the large ones) and encouraged him to use those, instead of having to "rush" to the bathroom which can hinder the progress of going.
Since I was a stay at home mother, I could keep a constant eye on him and would grab the bowl and place it under him whenever I saw the urge coming on--this took some of the pressure off of him of not having his feet securely planted on the floor, unlike sitting on the toilet where his feet just dangled and didn't have a secure platform to help him push.
Then I also taught him, since it was "his" poop, it was "his" responsibility to take care of it. So when he was done going, I would have him pick up the bowl, dump it in the toilet, wipe, throw the bucket away, flush, and thoroughly wash his hands.
This seemed to transition him nicely to going to the bathroom when the urge came on. I also like to think that the lack of pressure I put on these normal functions prevented him from becoming a "bed-wetter". It just takes some time and patience and lots of lots of love!
Sincerely yours,
M.