Contact Social Services and ask what steps need to be taken to have him involuntarily commited. They can usually commit him for 3 days, enough time to get a diagnosis and start him on medication. But since he is an adult he cannot be forced to take the medication or go to counseling. It might be possible to have him placed in a halfway house to continue treatment.
Schizophrenics can become violent so your aunt's safety should be your primary concern.