S.R.
My cousins (three of whom are deaf or hearing impaired) all went through a period of removing the aids. For one, they just were not helping her. Another didn't like the way they fit her. A third, I don't know that they ever really figured out the problem, which eventually resolved itself.
Like everyone's said, make sure the aids are actually helping.
Here's a possibility that's not been mentioned yet - many kids have a rough time adjusting to preschool and kindergarten - not so much in the typical "hard time adjusting" ways, but just in being SO TIRED when they're done for the day. A lot of kids come home and just need quiet time to balance out all the noise and activity of school.
Removing the hearing aids is a really sure-fire way of getting that silence. One cousin of mine, who is an adult now, still removes her hearing aids at the end of stressful days, to just give herself some silent time. I believe one other cousin (also an adult) turns his aids off any time he wants to concentrate.
If he leaves them in at school without much fuss, my guess is that they serve their purpose at school...
So perhaps the problem is not that he hates them, but maybe he just needs some de-stress time after school.