L.,
I wouldn't push your son just b/c DC wont advance him to the next class. If they want to leave him in the 2s... so be it, don't stress your son, don't stress yourself. Save this energy for things that REALLY matter!
I have learned this lesson w/my now 5 yr-o son, who went potty on his own until almost 4 and I just about lost it then....(not to discourage you, but this is when he finally went potty!) I thought he'd go to college in diapers!!! I'm not kidding, I'd cry I was so frustrated!
Luckly, our DC ctr was a lil' more cooperative and made ane exception w/him (although their rules were no diapers on the 3s class), which eased off a lil' of stress on me, but still I nagged my son like there was no tomorrow. My poor baby, I feel so guilty now! No bribes, no treats, no movies/cartoons, big-boy undies, no promise encouraged him to go... I quit diapers long before he actually went potty, at least 5 months, I 'roughed' it washing dirty underwear everyday!!! It got to the point where my son'd rinse his own undies.. it was CRAZY!!! He'd rather do that then go potty on his own. I couldn't understand, I was on my last string! THen one day, outta the clear blue, while we were eating dinner, he gets up from the table and dissappears.. I call out to find out where he is and he yells from the restroom, "I'm in the potty" and I come in, and yes he was! He went potty! I couldn't believe it, almost as he'd known to go all along.
So I promise, he will not go to Kinder in diapers... I'm expecting my 2nd and I know that when he is ready, he'll go. PERIOD.