I do not think they have any right to decide what you send in for him in
his own, clearly labeled sippy cup.  Fisher-price has a very nice one that
closes up when not in  use.  Or you  could send his milk in a thermos and
they can pour it into a sippy cup.  No different than any ABM or a child on
a special  formula.  My son  was allergic to  milk and soy and was still
drinking an awful concoction at 15 mo called Gerber's meat base formula (he
was 35 yesterday).  Would anyone have had the right to  tell  me what to
send to day care for him to drink?  I think not.  Simply tell them  you
will send his milk in an appropriatelt labelled container and they Will
serve it period.  No argument, not their business.  Sincerely, Pat in SNJ