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My 4 1/2 year old son had many of the same symptoms - diarrhea,
slow-growth (not really abnormal, but almost always well below 50th
percentile).  His bowel movements were generally very runny, yellow,
and frothy.  He was given the same types of tests in the late spring
of this year and the results were all negative.

Based on the symptoms and the fact that when we removed gluten from
my sons diet and saw a marked improvement in his bowel movements,
our doctor gave us a provisional diagnosis of Celiac Sprue.  He
recommended that we keep my son on a gluten free diet for 6 months
and then do a challenge, reintroducing gluten, to see what happens.
If the results still seem to indicate gluten intolerance, he said, he
is willing to do biopsy tests, which to as I understand it is the only
way to be sure of a Celiac Sprue diagnosis.  He also said that the
blood tests were NOT completely reliable when the results were negative.

Since the late spring/early summer when we removed gluten from my sons
diet we have seen a marked improvement in both his bowel movements and
his growth.  At one point in late August/early September we accidently
gave him some gluten (a mix-up in the freezer) and the disturbing bowel
movements returned.  The six months aren't up yet, but given the pattern
we have seen plus the fact that my niece also has Celiac Sprue, I
strongly suspect that he does in fact have Celiac Sprue

My son has also shown signs of Lactose Intolerance.  That is what we
thought the problem was at first, since I have it and because he showed
some improvements at first when we removed lactose from his diet.
Lactose Intolerance is a common red herring for those suffering from
Celiac Sprue.  I have a copy of a research study which indicates that
one of the effects of Celiac Sprue is to reduce the amount of lactase
produced by the patients body.  This side effect could last up to 4
months after the removal of gluten from the diet according to the
research study.

I suggest you try removing gluten from your sons diet and see what
happens.  After all what could it hurt?  Unfortunately, I have heard
from a number of people that doctors are often resistent believing in
dietary related disorders and you may have to "prove" your case.

Steve Greenspan