First of all, if he's having a negative reaction to all bright light, this is something that you should consult your vet over. It can be a symptom, believe it or not - photophobia.
However, if it's just the strange light that's bothering him, try creating a positive association with the reflections for him. Get him to sit and watch you, and have some chicken, or some other treat he really loves, ready to hand. Ask a friend to create a reflection on the wall or ceiling with a watch or mirror. While your dog remains calm, sitting and watching you, praise him and give him chicken