My son's landlord claims that they can ban a certain person from providing special needs services to my son in the apartment in which he lives. This person has not been convicted of a crime... does not disturb other tenants. Can a landlord do such a thing?

The landlord can ban someone from your son's apartment only if: (1) the landlord lives in the same building or nearby and has a restraining order against that person; or (2) the lease gives them that right. Read the lease and see if it says anything about the landlord having the right to ban certain people from the property.

There has to be some reason, you failed to say why the landlord wants to do this

A little more detail might help. But if a physician has ordered special care and the landlord blocks it, there might be grounds for a suit.