There's actually a lot of humor in this situation, and you can use that. "You like being a baby? Guess what? You're now a baby! Tomorrow I'll dress you, and you'll eat baby food, and you'll be with ME all day long because that's what babies do. Oh - look at the clock - it's time for babies to be in bed..."
Follow through on this! Your smart son will see through any shilly-shallying on your part. Feed him baby food with him buckled in a high chair and with you holding the spoon. Have him drink from a bottle. Put him down for a morning nap. Carry him or push him in a stroller - don't let him walk. Don't let him make any decisions.
This is NOT punishment, so check your attitude. You're giving your son something he wants; this is a mama-baby day. Be friendly, firm, and a little funny. Don't try to reason with him or "teach him a lesson." Don't say anything more about school next year, and don't talk about it to anyone else when he's around.
After the novelty wears off, he will surely get bored with the baby treatment (it might take from a couple of hours to a couple of days!), and then it's, "Well, then, you're making the switch from baby to big boy. There's no in between. The diapers are a thing of the past."
Your son's saying, "I don't care" (like Pierre in the old Sendak rhyme) is your lead here. I have a feeling he might really be saying, "I am a little scared of what being a big boy may bring - school and all that - so I think I'll hold onto what I have." And he's getting a lot of mileage out of this situation, isn't he?
I just noticed that a couple of other posters suggested this same sort of thing. So I'm not being original after all. :^)
(By the way, everybody likes to be "babied" sometimes... and I have a photo of all my children the day they ALL decided to be babies and wanted to drink out of bottles. The oldest was six or seven. That lasted a whole afternoon.)