The easiest solution would be to purchase heavier (i.e.; light blocking) drapes for his bedroom windows - and maybe a nightlight. Turn closing the drapes part of his night time routine. Close the drapes, take a bath, put your jammies on, etc. You can always enter his room when you wake and open the drapes to let in the morning light. But, while he should be asleep, keep the drapes drawn. No more shadows. No more floodlights.
In light of the comment exchange (below), let me offer another idea: Shadow Games
Shadow games provide a way to turn something scary into something fun. Essentially you use your hands and a nearby light source to throw shadows up on the wall. Here is a website that explains Hand Shadow Puppets better than I can with words.
My son had a similar issue when he wasn't much older than yours. I tried to explain the concept of shadows and we played a bit with the hand puppet idea over the next couple of nights. It made a huge difference for us!
Please don't be concerned that you passed a phobia down to your son. I feel confident that he is frightened simply because he doesn't understand what a shadow is and where it comes from.