Yes, its true. You can become allergic to the sun, but I've heard that kids can grow out of it after a while. I will keep my fingers crossed and send good thoughts for you and your son. I'm sure that when you get in to see an allergist he can give you some tips to help prevent the rash.
BTW - I have the opposite problem. I am allergic to the cold! Yep, I couldn't believe it either, but I always break out in awful hives once the temperature went down on hiking trips. I finally found out that it is an allergic reaction to cold. Well, I'm not letting it stop me from going outside. I just have to take preemptive measures to limit its occurrence, including putting on my puffy down pants long before I think i need them