Zachary Crehan, 35, was last seen near a hiking trail on Jan. 5.
Rescue teams have searched the area, but have not yielded any results.
Crews searching for hiker who went missing near Mount Wilson trail
Crehan's brother says Zachary was not an avid hiker, which makes his family even more concerned.
"I don't think my brother was someone that would be able to get himself out of a mess up in the mountains," said Jacen Crehan. "He didn't go there very often so I don't know that he would be able to be prepared. And in fact, we know that he wasn't prepared for it. He was in a short-sleeved shirt and I believe shorts."
Anyone with information about his whereabouts is asked to call the Los Angeles County Sheriff's Department.