I can’t believe that they would be allowed to exclude a child based on a medical condition (GRRRR!!!).  I would definitely press the issue under the ADA Act, especially if they receive any sort of state or federal funding, which many daycares do these days, in the form of subsidized care.  I can see how they might be able to avoid administering meds if they don’t have a nurse or someone similar on staff, but they should not be able to refuse to care for your son.  Good luck with this!