Sean Sidi went to visit a former high school teacher at 150 Oak St. in San Francisco around 11:40 a.m. on Tuesday, according to family members. His visit was recorded by surveillance cameras.
He spoke to his father by cellphone around 1:30 p.m., and told him he was catching the bus to "the park." His phone went dead around 2 p.m. and he has not been heard from since.
Sidi is considered at risk due to his medical condition, according to police.
Family members say Sidi has always told them where he is and where he's going, rarely leaves the house for long periods and is usually home by 6 p.m.
Sidi is described as being a slim male of mixed Caucasian and Asian descent, with dark brown hair and eyes and braces on his teeth. He is 5 feet 5 inches tall and weighs 120 pounds.
He was last seen wearing a black and gray North Face hooded rain jacket, blue jeans and black canvas shoes. He recently came out of an arm cast and was wearing a black splint on his left wrist and arm.
Anyone who thinks they may have information on his whereabouts should call San Francisco police Sgt. Ann MacKenzie at (415) 734-3268 or go to www.seansidi.com.