Kelly,
My 5-year-old Corey has anaphylactic allergy to milk and he's always been just
slightly under the norm in weight for his age. But not anything alarming.
In fact, my 12-year-old son was under the norm until recently and has now
evened out -- he was never allergic to any foods.

Luckily, Corey is only allergic to milk (and environmental allergens -- dust,
mold, dander); so he can eat peanut products (which contain the fat that was
especially important when he was a toddler). Olive oil is another good way to
get "healthy" fat into his food. His diet has been pretty balanced all along.
I don't know much about kidney problems, so I'm not sure if that's where the
weight problems come from. What does your doctor have to say? Is he or she
concerned?
And have you talked with a nutritionist?

Good luck!
Rose