Some children really don't mind about having a soiled nappy. It has nothing to do with intelligence, and in your DS' case I think he knows when he needs to poo before he does it. I suspect that actually he quite likes it, and is attached to his poo as it is a part of him. So this is why he denies doing it, I think. He certainly knows it bugs you, but really can't see what the big deal is; I guess he doesn't care about potty training, because he doesn't think it applies to him as you just put another nappy on and that's the way things are. And adults using the toilet has nothing to do with a little boy doing the same to his way of thinking.
I do think that some children need to be pro-actively trained, and that they can become conditioned to use nappies. It would be good to start with some pre-training, getting him used to seeing other children use the potty and toilet, buying potties and loo seats and new pants, reading books about potty and loo using, and all the time giving a clear message that this is what we all do. Is he your first? Sometimes it can be harder for only children unless they go to a nursery and see other children using loos and potties regularly.
When you do start, don't use pull-ups. He will just view them as a new type of nappy. Plan to stay close to home, and if the weather is warm you can be in the garden with the potty and accidents are less of a problem. When you do venture out, loose shorts with elasticated waists can be worn without pants underneath, OK they won't absorb an accident, but it's one less item to wash! Those swim shorts are good too, and quick to dry if you're running out of clothes.
The main thing about potty training I found, was to have a philosophical approach. It's a new skill to be learned, like dressing or using cutlery. It isn't that much of a problem if children put on clothes back to front or bits of food get knocked off their plates, so wet and soiled clothes are bound to happen, and we just take steps to protect the sofa and get a good carpet cleaning spray. DS will pick up on any tension, so you need an almost zen-like calm and a matter-of-fact attitude.
Hope this helps!