Re: WOAH! Celebration & Suggestions Needed...
Don't be pushy.
I think that PLing is such an awesome time! They take such pride in going where the big people go. Expect setbacks. Expect good days & bad days. Always praise, never punish.
What I might do is put him in undies during the day and see how he does. I call it "testing". If he's indeed ready to make the transition, you should see him caring enough to go or ask to be taken before or right after an accident. He'll care that he's dry & clean. If not, he'll just use the undies like a diaper. Then you try again in a month or so and see if he's ready for undies yet.
No biggie if he's not. Yippie if he is!
Our son does get an M&M or sticker for going IN the potty. Nothing for accidents besides a set of dry clothes and a reminder that pee/poo goes in the potty.
But our guy eats up praise. And praise is free(and not bad for your teeth!!).