<<Disclaimer: Verify this information before applying it to your situation.>>

Just a comment on the length of time for the villi to show damage:

It takes awhile before the villi at the top of the small intestines begin to
flatten.  Then, as they flatten and absorb less, the food moves further down
the intestines and begins to damage the next batch of villi.  In other words,
the villi closest to the stomach get damaged long before those in the middle
of the small intestines begin to get damaged.  And villi at the end of the
small intestines take a L-O-N-G time before they start to show any signs of
damage.

When the doctor goes in for a biopsy, he takes one or two "samples" from
heaven only knows where.  And those samples may or may not be "good" for a
diagnosis.  (Does this start to give you an idea of how really unscientific
and unspecific these tests can be?  Scarey, isn't it!!!)

So when you put someone back on gluten, it can take awhile (sometimes quite
awhile) before there is enough damage for the sample to be pretty sure to
catch some of it.  People who go on for just a few weeks are so disappointed
when the biopsy comes back normal.  And is that proof that they aren't
celiacs?  Heck no!  But the darned doctors don't always know how to handle
this.

The proof is in what you are going through.  The doctor you saw first
"decided" that your son was a celiac.  And he might well be.  But taking him
off of gluten without using the only gold standard test there is was doing
the poor kid a big disservice.  And now you are having to go back and start
all over again.

What a shame.  And your poor son.  (Not to mention poor you, having to deal
with all of this!)  I hope everything works out well and that a diagnosis is
not long in coming.  But I suspect that six months is probably close to the
minimum that you'd want him on gluten before doing a biopsy.  It really does
take awhile for the damage to show up and, as I said, you want enough damage
to be there for the biopsy to pick up on it.  You certainly don't want a
biopsy that the doctor says is normal (if your son is a celiac) and then have
to deal with that mess.

  ***
I recently had an intestinal biopsy after being gluten free for two years
(after being diagnosed with DH).

I was told to eat gluten for 9 days before the test.  My biopsy came back
with moderate villous atrophy, and other inflammatory changes indicative of
subclinical Celiac disease.  I think everyone reacts in a different way.  My
only symptom was nausea after the 6th day and I never got my rash.  I really
think 6 months is an awful long time to be eating gluten especially if your
son gets so ill.  Why don't you ask if he can eat gluten for a shorter time,
(perhaps a few weeks) have the biopsy done and if it is not conclusive have
him eat gluten longer.  I think while he is having the horrible diarrhea he
should be monitored for dehydration and perhaps even given some IV therapy.