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

generally speaking, I do not beleive that a DTP injection, even if it contained
gluten, could be harmful.  The serum is injected - usually into the thigh here
in Canada - and it is my understanding that the gluten has to enter the
digestive track to cause damage.  I would check with the doctor to clarify
if the injection could result in intestinal damage.

It is possible that your son had a reaction from the flu shot, not because of
any gluten, but maybe from another allergic reaction.  It is not uncommon for
people to have reactions to vaccines which are egg based, as the flu shot is
atleast here in Canada.

This ofcourse, is only my opinion and I would suggest that you clarify these
points with a physician.

Hope this is helpful,
Deanna