Amaobi Obiorah was last seen at around 7:30 p.m. Friday at his home on Creditview Road, Mississauga.
Police and his family are concerned for his safety because of a medical condition that leaves him functioning at a younger age.
Obiorah is described as black, six feet one inches tall, weighing 140 pounds, with short black hair. He was last seen wearing orange basketball shorts, a grey T-shirt and black, white and red Adidas running shoes.
He has no cell phone and no access to money.
Obiorah was also reported missing earlier this week, and was located in the Toronto area.
Anyone with information is asked to call investigators at the 11 Division Criminal Investigations Bureau at (905)453-2121, ext. 1133. Information can also be left anonymously at Peel Crime Stoppers at 1-800-222-TIPS (8477).