It's hard to say based off that picture. But I am assuming theres a fungus growing on his head and thats why its completely white? It looks a bit like what my guppy had which is columaris fungus, also sometimes known as hole in head. I would try treating your fish with Maracyn. Just follow the directions on there and hope it isnt to late. You will need to treat the whole tank because it spreads very quickly, also either throw out and repace all fish nets youve used or bleach them so it cant spread to another tank.
Something I learned about it is that it can spread fast and slow, normally when spreading quickly (usually when it appears on the fins or body) the treatement doesnt take affect soon enough for all fish. As for the slow spreading cases (like on the head) there is always a chance depending on how quickly you get it..
I cant say for a fact this is what it is I am just letting you know because it does in fact look like what I lost 5 guppies to. 3 which had the same on their head and 2 which ended up getting it near their fins. Clean your tank extreamly well after treatment. Good luck and I hope I helped!
