My four year old wont eat, what should I do?

My four year old became ill on Christmas Eve, and vomited twice. Atferward he wasn't ill for the rest of the day so my husband and I just figured that he had too many sweets.
However, It is now the 29th and he still wont eat. Every time we try to make him eat he just says that his tummy hurts. He has had no fever and no more vomiting. However he is also very tired and taking voluntary naps. Today when I came home for my break I saw that he looks pale and has dark circles around his eyes. He wont eat anything, Ive even tried all his favorite foods, waffles, sphagetti, grilled cheese. He takes two bites then says his tummy hurts. We normally cant get him to stop eating. I am worried, what can this be, and what should I do? Also, two days ago he complained a little while of a headache, but he hasnt mentioned it since. Someone please help, I am very worried.