This reminds me of my first DD whom we called the "bottomless pit."
When she started at her new daycare at 19 months old, I told her teacher that when I picked her up at 5pm, she was thirsty and by the time we got home she was freaking-out hungry. (If I didn't have left-overs that I could just toss on the table with no prep, I was screwed!) Her teacher just kept describing how much she would eat. She would always come to the table immediately and she would eat and eat and she would be the last one finished. I reasurred her that I believed they were feeding Alex as much as any of the other children, but that Alex was STILL hungry. We started sending extra food (yogurt to add to the normal 3pm snack) to supplement her diet. It worked. I could at least have a breather when I got home.
In the evenings and on weekends, we would sometimes just keep feeding her until she wouldn't eat anymore. It was kind of fun. This all lasted from about 17 months to 28 (?) months. She is now a normal picky toddler. Alex was also nurisng a little during this time, but she never had loose stools. Once solids were introduced at 6 months, she had very few loose stools.
There are some conditions (like celiac disease) where nutirents are not being absorbed, but these aren't very common. If I were you, I would include a good quality protein with all snacks/meals and wait and see. If no change, do a little research on the pediatricians available in your area and try to find one that is more natural parenting/ homeopathic friendly and see what he/she has to say. You might also want to kep a food diary. This will be especially helpful if you decide to talk to a pedi about your DS.
Good Luck! I hope he doesn't break your grocery budget!