How old is he? If you think he's teething, or it's a toothache, you could try putting a bit of orajell on his gums. (They don't like the taste, but it really helps.) Otherwise, find out if his throat is swollen. He needs nutrients, so if he won't eat or drink, at least syringe him some pedialyte or karo syrop mixed with water.
Good Luck!
Valerie