Potty Training...a 3 yr old...

So, I am currently potty training 2 toddlers.
It was never my plan to be potty training a 3 yr old. My daughter is 18 mos and so she is right at the perfect age to learn. My son, however, I introduced him to the potty just after his 1st birthday and he loved sitting there. After a few months or so of that I had him actually trying to go potty. He would go every once in a while and would be so proud of himself and loved "shooting" the cheerios. After the age of 2, I started taking him more often and for 4 days straight he stayed dry wearing 'big boy underpants', however the 5th day he absolutely refused to go.
Now, he will be 3 soon and some days will do very well and have very few accidents, but the next day he will refuse to go again. I have tried several different things.
I have tried: taking him every 15 min, I have tried rewarding him with treats/toys/candy when he goes #2, I have tried punishment if he tells me he doesn't have to go and then messes his pants, I have tried getting him 'big boy underwear' with his favorite cartoon characters on them, I have tried rewarding him for staying dry and clean, I have tried explaining to him how to check his pants every so often to make sure he stays dry, I have tried celebrating with "Potty Day!", I have tried letting him hold a mirror and make funny faces while he sits on the potty, I have tried reading to him and singing while he's on the potty, I have tried letting him do the whole potty thing without help, and I have tried stepping out and closing the door to give him privacy while he goes....nothing seems to be working...


Does anyone have any advice/solutions/suggestions?
What worked for your kids?