It's no wonder kids today don't really care about wearing diapers even when wet because the diapers today are so absorbent that the kid is unlikely to feel wetness and only feels the swelling of the diaper.
When you change his diaper next when wet put your finger onto the absorbent part of the diaper especially if it's Luvs, Huggies or Pampers and feel for wetness.
Your finger will likely not feel the wetness of the diaper and that's what your son feels.
Your son has no incentive to be in dry underwear because he's already basically dry but just wearing a swollen diaper.
Now for the poop yes some kids enjoy the feeling and texture of the poop on their bottom so that could be another reason.
What you need to do is remove the diaper from him and never put another diaper on him again and just put him in regular underwear or those cloth training pants where he'll feel wet.
When he does feel wet then he'll have more of an incentive to potty train.
When I was a baby I wore cloth diapers and was potty trained at 18 months because the old cloth diapers actually felt wet and not dry like the disposable diapers today.
So for those parents who worry about a wet diaper at night it's really no problem because the baby or toddler isn't really feeling the wetness anyway because it's locked into the SAP super absorbent polymere which is what those diaper crystals are.