Even if school is manageable without ear plugs, it would be best if he used them until he has no need in the slightest for them. The occasional overload (things speeding up) means his brain is still working harder than it should.

It is worthwhile for him to ask his friends and teachers how his speaking sounds when things speed up. This can be a sign of continuing problems. The brain can slow down and make it appear than his environment is speeding up. When this happens, his speaking will likely become slowed and labored, maybe even struggle to find words.

I have had episodes like this. I have not been aware of the slowed speech but others have.

My best to you.