Thomas Riley, 16, was killed Feb. 27 outside his home at an apartment complex near the intersection of 105th Street and Western Avenue in the Westmont area of South Los Angeles.
Detectives and the victim's family hope the donated billboard space will lead to an arrest.
"Thomas was my baby boy," said his mother, Victoria Winston. "I just want… all families to know you don't know the loss until it happens to you."
Detectives said Riley was standing in front of the complex with friends when they were approached by two male suspects who asked who asked about his gang affiliation.
Before Riley or anyone else could answer, one of the suspects pulled out a gun and began shooting. Riley was shot five times and another victim was shot once.
"This wasn't a case where he had anything to do with the circumstances. He was a victim in the ultimate sense of the word," said Lt. Mike Rosson of the Los Angeles County Sheriff's Department.
"It's not fair for somebody to just come up and take his life because he wasn't in a gang," Winston said. "You didn't even give him an opportunity to say he wasn't in a gang."
Detectives said they believe the suspects were gang members who mistook Riley and others with him for rival gang members.
Anyone with information is asked to call detectives at (800) 222-8477