He may not actually be quite ready yet. It's not worth really fighting him to use the potty. I say just put the diapers back on and try later.
Some kids don't potty train until almost 5 years old and that's okay. All kids are different. You'll actually miss having your kid in diapers when they get into their teens.
If you really want to get him out of diapers though you have to just leave the diaper off for good and put him in underwear. He'll have wet underwear and even poop filled underwear and messes to clean up off the floor but he'll eventually get the hang of the potty.
You never go back to diapers when potty training a kid as it confuses them.