A white background usually helps from my experience. Put a piece of white paper behind your fish's tank for about an hour and see how he responds.
Also, try having the tank's light on at the same time the lights are on outside of it.
It's the different amount of light on either end that causes him to see his reflection. If the light is nearly equal on both ends it shouldn't be a problem. So far I've heard you say you've tried having his light off with an outside source on and he still flares, or his light on with outside light off. Again, hopefully you'll be able to even out the light on both ends at once and be able to keep him from flaring.
If not, I still stress the white sheet of paper. I've found it's worked for all of mine and my sister's fish.