Actually, he is doing a lot better now.It is hard to say as you have little information. How old is the fish and are there any abnormal / different looking features to it?
Head down sometimes indicates swim bladder disorder. If you have had the fish for some time and have not had an unexpected cool down in water temp that is likely not it. You should look at the fish from the top of the tank down. If you see scales sticking out, like a pine cone it is dropsey and the fish needs to be put down. They only other thing I can think of would be an intestinal upset that has something making the fish front heavy.