First of all, please don't be afraid to call your doctor if you are concerned. The nurse can ask you questions to help determine whether this is just a normal thing (which is my guess) or something to be concerned about. Remember, they've been asked it all, so they will understand your concern and be there to help.
Toddlers are funny creatures. Some days it can seem like they survive on nothing but the air they breath. Other days it can seem like no amount of food is every going to be enough to satisfy their hunger. It's crazy, and I never found a pattern to it in my boys. I tried very hard to just offer healthy food and relax as they ate what they wanted and then were eager to go play again.
My best advice - keep offering food every couple of hours and try not to worry if he is not interested. Do make sure he gets plenty to drink, but try (and I do mean try because it is not always easy) to trust that he is listening to his body. His body will let him know when he's hungry. If he's not hungry, then he's not hungry. If you try to force him to eat when he's not hungry, he will learn to not listen to his body telling him he's not hungry, and that's not something you want. (That's a problem far too many adults, including myself, do not do well enough and I could definitely stand to lose a few pounds.)