Knowing a little about learning difficulties, I'd say take a doctors note and as much information as you can.
Aural sensitivity is no fun, I have it when it comes to very loud, sudden noises. Bonfire night is no fun for me and I use the squishy in ear ear defenders to ease the noise. I can still hear speech, and I would assume this is the case with the over the ear kind of ear defenders.
As a passenger, I'd rather have a calm, happy child on a flight, than a child who might possibly become aggressive due to overstimulation (no idea if that's true of your son OP, but it is the case for some, I think).
BUT, I can see the crew perspective too. If it were me, I'd try and phone someone, or use an airline where I know this wouldn't be a problem.
PS: Most ear defenders, to my knowledge, aren't electrical.