I've also found places to be very accommodating in heating up a couple slices of pizza we brought in a plastic container in their employee lounge.  (I prefer a big container so they keep the pizza inside it.)  Even though DS has soy cheese, he prefers his pizza cheese free, so his pizza obviously looks different even. 

Many of these places had no outside food policies, but they gave us no problem whatsoever when we needed to bring in our own pizza or cake.  They all realized they had no food they could serve him with a milk allergy and we were potential future party bookers ourselves.  Sometimes we were.  We always mentioned it when we arrived to the party host - so if there had ever been a problem we would have arranged an alternative before food time - assuming we ever needed to make it past an ADA explanation. 

FYI, school also stores his pizza (and ice cream when applicable) in the staff lounge and heats his pizza up for class parties.  Sometimes a teacher happens to have a mini fridge and microwave in the classroom also.

That's what I do, too.  It works well enough. 

I microwave the pizza really really hot and then wrap in foil.  DS does not mind eating it a bit cool.  I have recently started make in "special cold pizza" and sending that into school.  And as long as it is the "special cold pizza Mumma makes" he is find with it being cold...

Now that DS has decided he likes pizza, he wants to have it at the bday parties he goes to.  Around here most party places serve pizza.  In the past DS was fine with safe chicken nuggets in his thermos.  I can try to wrap the pizza in foil and put it in an insulated bag.  Do you think that would work?  Are there any flat type containers that work like thermoses?  Just wondering if anyone here has any suggestions.  I have a feeling that after it sits in the insulated bag for an hour DS will not eat it bc its not fresh from the oven.