What are you using on the floor? Newspaper?
Just some handy little tips to offer, you can try use other flooring that will not keep the tank warmer but help keep your snake warm. You can use carpet - not the soft wooly type ones but something a bit like marine carpet will help him stay warm. Another that I have used and doesn't seem to have any ill effects is sugarcane mulch. I presume any mulch would probably work but I have only used the sugarcane one. Keeps them up off the cold floor. Newspaper while being the most preferred substrate can take up the cold floor and not offer any insulation, unless of course you use a thicker layer of newspaper rather than just a couple of pages.
Does he have a hide box close to the warm end? Make sure his hide is only just big enough for him. When snakes go away to hide they like confined spots so that they can coil up into a tight ball. When they can coil up like this it helps them retain the heat they got while they were out basking. The larger the hidebox the more area for his heat to escape him.
Someone has already mentioned using a heatmat underneath the enclosure. Using a heatmat as well as your light will do wonders in bringing up the heat. I have above and below heating on a few of my snakes atm. The air temperature is just so darn cold atm, even that fights to warm my snakes properly but still much better than it is with just one or the other. I have thickened up the newspaper padding on the floor, and added square cut-outs of carpet under others, just to get them up off the cold floor.
I am not sure about using foil. It would be awfully glarey for your snake. You can get some nice foam backgrounnds for reptile enclosures. Foam would be my preferred choice than using foil. If the top of the tank is all open gauze, cover half the top which will also help keep some of the heat in.
