I have a girl instead of a boy, in case that makes a difference, but we didn't train her until 3 years 8 months. Some of it was us dealing with the stress of my mother being ill and then being in hospice care for many months when DD was 2, then finally passing away just before DD's 3rd birthday. And some of it was just DD being super-stubborn and strong-willed, screaming that she was afraid of the potty and refusing to have anything to do with it - despite incentives like candy, stickers, and all that. She was able to do 3 year old preschool in pull-ups and never had any accidents, but I knew 4 year old preschool would be a different story and that physically she was fully capable. So when I had a weekend off from work when I would be home, I just decided that I was done talking about it, the diapers got tossed, and DD got underpants put on her, and that was that. DD flipped out, threw a fit, cried and screamed that she wanted her diapers back, and I held firm and said no way. She was going to have to figure this out and we would just clean up whatever messes she made. We went through several wet panties that day. She got a pull-up at night. The next day she woke up dry, but refused to sit on the potty and try to go. On went the underpants. She freaked out again, and again I stood firm. It was 2 more hours before she started dancing around and realizing that something was about to happen, and I dragged her over to the potty and made her sit on it. She started crying and yelling again, but couldn't hold onto the pee anymore. In it went into the potty, and that was that. She never had a pee accident after that, and never wore a diaper again. She needed to feel what it felt like to be wet and realize that she didn't like it. She still had pull-ups at night, for about a month, but then kept waking up dry, so they quickly got ditched. We rewarded her each time she went, until it became clear that she didn't need it any longer, and I told her she was a big enough girl to just be able to go potty on her own, and not end up messing herself.
Poop was a different story. She was still pretty stubborn about it, went through bouts of being constipated because she was afraid to go and it would hurt, having to do Miralax, etc. She got much better about it the summer before she turned 5 - that took having her sit on the potty twice a day for 10 minutes with my smartphone, to give the poop a chance to come out. She started kindergarten and goes poop at school no problem, wipes herself, and usually has a BM every day. She doesn't need any help or reminding from us- THAT to me is the hallmark of being "potty trained." Being fully independent, or at least needing minimal help (i.e. wiping their bottom well).
It's possible that your son just doesn't care about the rewards, the praise, and pleasing you - some regression at this age is pretty normal. You might have more success if you let him go back to wearing diapers, then try again when he is 3 or 3.5. To me, incentives help, but in the end, it should be about them doing what needs to be done simply because it is what is expected of them - not just when it comes to potty training, but any good behavior you want from them. He should want to use the potty because it is what big boys do, and he wants to feel like he is fully capable of keeping himself clean and managing his own body's needs. The motivation should come from within. And that might be more easily accomplished when he's a little older and more mature. I would rather have my kid in diapers and wait it out until they seem more ready, then have them potty train relatively quickly, than go through what Sherri G.'s friend went through, dragging it out for months and months, always having accidents, but still insisting they are trained.