My son eats the thin crust pizza with no cheese, just sauce, pep, onion 
and green pepper on the crust, when they have school functions that 
provide pizza.  He loves pizza with no cheese and considers it a big 
treat.  Dominos will make their personal size pizza with a thin crust if 
you explain why.  They have told me there is no milk in their thin crust 
when I called their offices.  My husband told me he read a brochure that 
did not list any actual milk ingredient, but stated that it had milk.  My 
son has never reacted to it.  My son used to be extrememly sensitive to 
trace amounts, but now at thirteen doesn't seem as sensitive.

I tell you this because I would not want to mislead you.  You know your 
son best.  I would call their offices and see if you can find a comfort 
level.  There have been times I have made a small personal pizza and just 
taken it in to the school myself.