It's frustrating, but it can be completely normal. Is this child "normal" and on target in every other area? Speech, gross motor skills, comprehension, and so on? Then truthfully, I wouldn't worry. It can be more common in boys, but it can occur in anyone.
My son was like this - just didn't get the "signal" from the bladder to the brain. I did everything you're probably doing - sticker charts and M&Ms & incentives and constantly putting him on the toilet, even wetness alarms on his underwear & pajamas. It made me so frustrated and I tried everything - until I spoke to doctors and preschool teachers and found out he was normal. Part of it could have been that he was just too interested in everything that he was doing and everything that was going on in the world, and he didn't care about the toilet! But the rest of it was truly developmental. It was nothing I was doing or not doing.
In my son's case, even after he was daytime potty trained, it continued at night for many years - that's called nocturnal enuresis which means "nighttime peeing" basically! It's not serious, very common in boys, and I can give you more info on it if that's a problem too. Our pediatrician and a wonderful pediatric urologist gave us great info Very easy to manage. The important thing is not to shame the child or try to force them to do things their bodies just can't do.
Leave it alone, unless there are other issues. It could be stubbornness but it could be completely developmental and nothing you can "train" him/her to do.
Sometimes they get the hang of it in preschool when they see other kids using the toilet, particularly when the whole group goes at one time. But not always.
Other than the expense of diapers, siblings who tease him, and some thoughtless remarks from other people who assume you are doing something wrong, there's no down side to this. I promise you, he will not go to 2nd grade in diapers! He will not graduate from high school in Pull Ups. I know it is frustrating but try not to let it bother you - the child will pick that up and it just doesn't help at all.
Let me know if you want more info. Hang in there!