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

You should not change diet until after the biopsy is completed.  You are
potentially giving away the opportunity for an accurate diagnosis if you go
gluten free prior to the biopsy.

If that has been done, or partially done already be sure to explain what has
been eliminated, and what types of foods your child has been eating before
the specialist does the biopsy.