How old is your dog & what breed? Does he have any known health issues?
Is he current on vaccinations & wormings?
What do you mean by he cant lay down his head? Does he appear to be in pain? How long has he been acting like this?
How long has i been since he last ate? Have you tried a bland diet such as boiled rice with a little chicken? Does he have any other symptoms, such as vomiting or diarrhea?