This is kinda what I'm hearing everywhere. "Don't worry unless the kid is unhealthy" "Oh, they are?" "Um, don't worry anyway."
We've had pretty good luck avoiding what was looking like a pretty scary situation for a bit, and I kinda think that's a good thing...
I know when we realized that he'd gotten to the point where he was unable to eat more than a couple of bites at a time because he just had'nt been eating more ever... I got a bit scared, genuinely scared.
We're lucky about no allergies here. I have severe allergies, so this is almost too good to be true

There MAY be other physical reasons why he eats so little, but I suspect they're just exascerbating the situation, and are not main causes, even if they're there. In any case, both issues have been looked at and specialists are "pretty certain" they aren't causing pain...
I, personally, really wish the varieties of allergic experience were better understood publicly. I've been refused service in restaurants one too many times when I was just trying to sort out what I could eat. I now no longer ask, most of the time, wich is risky for me.