DANBURY — A city resident faces sexual assault charges after a months-long investigation determined he abused an autistic man last year, authorities said Friday. Gordon Fuller, 69, was caring for the 45-year-old victim while the primary caregiver was away last August, according to the report. Fuller was arrested on a warrant this week and charged with one count each of first- and second-degree sexual assault. He’s being held on $100,000 bond and is scheduled to appear June 6 in state Superior Court in Danbury.