Thin, peeling toenail?

DS's big toe (only one affected) has white lines and appears very thin. When he lets me touch it, it appears so thin in one spot that I may be touching skin. Is this what a toe fungus is? He is in socks 24/7 b/c we have wood floors that are cold and he's in socks at night. Maybe he's sweating in them. It seems minor to call the doctor, but maybe I should. He does flinch a tiny bit when I touch it and tends to bite/pull at that foot/toe (I may just be imagining these things as it's hard to tell). Any BTDT experience?