gray area under foot

I have a 33 year old son with Austism, he cannot talk or understand enough for me to ask him if it hurts,I don't believe its painful because I asked him if his foot hurts, he smiled.. I noticed that behind his toes where the foot begins at the top, its a light gray color as if his skin is dying, could this be from him wearing white cotton socks and his feet are always sweating? if not then What could this be?