Michael Sam says he's not convinced his older brother is dead after going missing in 1998 -- and now he's contacted a famous psychic to find out once and for all.

Sam met with Tyler Henry -- star of "Hollywood Medium" -- and explained the situation ... saying he was only 9-years-old when Julian Sam (a teenager at the time) mysteriously disappeared. 

Sam says his family pronounced Julian dead two years later -- but they never found his body ... and Michael says he believes there's a chance he's still alive somewhere. 

Enter the psychic ... who says he tried to make contact with Julian on the other side -- but got NO feelings ... saying, "It is possible that he might still be alive."

Sam says the reading gave him "hope" -- and says he vows to continue the search.