I see I left out some needed detail.
My Dad has a Sony Surround receiver. He finds the sound fine most of the time but as we all know some movies have a tendency to get quiet and then suddenly loud. He also doesn't like wearing headphones so that's not an option especially since the loud sound actually bothers him so headphones would be similar.

BestBang4thebuck - interesting ideas. The VCR trick is cool but I think he would lose his 5.1 processing.

Gonk - Actually he does have a receiver and he is happy with it except for the lack of this feature so he doesn't want to buy a new receiver for just this feature.

garcianc2003 - Headphones were my first suggestion since he originally told me he didn't like the increased volume at night when my Mom was sleeping. But he doesn't like using headphones and realized that the changes in volume actually annoy him all the time not just at night. Your suggestion to trim the speakers is good however the problem is he would have to trim them back to normal settings for most DVD's. After I set up his receiver I don't think he has ever adjusted the settings ;-)

I am really looking for a device which would connect between the DVD player and the receiver and limit the sound. Not sure if this exists.