Originally Posted by Blackcat31:
Maybe it's not an allergy to anything but more of a lack of some thing.
Sounds like there is not enough fiber in his diet.
Maybe have the parents track his food intake so that you can "see"/measure the amount of fiber he is getting.
Fiber does wonders for making firm BM's. 
Yes!
My family is vegetarian, mostly vegan, and though I sometimes serve chicken to DCKs, we all mostly easy veggies and fruits, nuts and beans. If I've had a particularly berry-licious and nutty day, I pay for it later in nasty diapers (ewww!).
Fruits and seeds/nuts are awesome, but there needs to be other, more "stool-firming" food in the diet. MunchkinWrangler suggested what I would have suggested: the BRAT diet.
It doesn't sound like "sick" poop to me, just really loose poop that can be firmed up with a couple dietary changes. Good luck!