OMG! I could have written that post about my 5yo DS!!! We joke that he must be feeding an entire community of tapeworms! He is just as you described -- he will literally tell me that he is hungry as he is walking out of the kitchen after dinner, or as we are leaving an all-you-can-eat buffet where he had three plates of food. He will ask me, and DH, and my other children, as we're serving ourselves dinner, "You're not going to eat it all, right? You'll leave some?" And then if someone should leave some of their meal on their plate, he will ask to finish it. I even have had to physically restrain him from taking food off his sister's plate, when it's something he especially loves -- she eats slower than he does, and he is convinced that she must be finished when he is, so obviously she doesn't want whatever is left on her plate.
As I mentioned in another thread yesterday, he has our city landmarked by the availability of food. You know, there's the cookie store. Let's go to the park by the pizza place. We're coming up on the grocery store that sells the green juice, Mom. There're the fairgrounds where we got that really good icecream (it was homemade) at the fair last year, Mom. He organizes everything we do by when and what he will eat. He also accounts for about 3/8 of my grocery budget -- the other 5/8 feeds four people!
I, too, rarely buy treats. But he has a memory like you would not believe, and can remember what we ate on vacation in Florida three years ago. Seriously! When he was two! He can remember where we've purchased any single item of food that he liked. So he knows where the cookie store is because his preschool class visited there and helped them make cookies last year.
Honestly, I don't know what to tell you about being worried. My DS is skinny, has always been skinny, but is extremely healthy and has had consistent growth. (He's got the same arc on the weight chart since he was about 18 months.) It worries me, because I don't know where this obsession comes from and I don't know if it's going to lead to eating disorder issues later on. I have always, always fed him whenever he has asked, even if that's every ten minutes, so he's never had reason to be so obsessed with when and how much he'll be able to eat. He drinks plenty of water, so I know it's not a hydration issue masking as hunger. And it's absolutely not boredom or a cry for attention -- we have a very attached relationship, I spend a lot of time with him, as do his father and brother and sister, and he is involved in a lot of activities that he enjoys, both inside and outside of the home. But things like taking food from his sister, and begging food from people whenever we go somewhere to visit, are really starting to be an issue for us.
In our case, this has been ongoing for DS's entire life. We've had him checked twice for GI issues, including Celiac disease, as our older son was diagnosed with that, and there is a genetic disposition. Our docs all say he is fine, including our very well respected homeopath and our holistic pediatrician. (We just found her, and she's fantastic, and I trust her more than I trust all of my other docs combined.)
So, I can offer commiseration, if not some answers. Hope you find those, though!