I know how your DS feels ... my DS hated the fireworks when we first took him to WDW at age 3 1/2. We'd always leave WDW before the fireworks started and we'd be down at the bus stop, watching them from there b/c they weren't as loud.
I can't recall what's open during
MNSSHP, but here are some suggestions ... maybe you can find something among them:
I don't know if they'll be running Country Bear Jamboree or Goofy's Jubilee thing ... but those would be two good places to be, I'd guess. You're indoors and things will be busy in there and the music, etc. should drown out the fireworks.
If the store in ToonTown is still open, go in there to escape them. It's across from Goofy's Barnstormer and there are characters usually there. While you might hear them, your DS will be safe inside knowing he's fine. Plus, he'll be distracted by all the Disney merchandise!
That's all I can come up w/offhand. Maybe someone else will have a good tip or two for you!?