Rukie Ware and Zainab Raji made their emotional plea as the Met police offered a £20,000 reward for information leading to the arrest and prosecution of those responsible for 21-year-old Ola Raji’s death.
Ms Raji said: "To many people Ola was just another boy or statistic but to us he was so much more, so much more.
"He is a son, brother and grandson and very much the heart of this family."
Ms Ware said: "We still don’t know what happened so if anyone does know anything that could help to give us a tiny fragment of peace that would be much appreciated, any knowledge they may have no matter how small they think it is."
Ola Raji was cycling onto the East Surrey Grove Estate in Peckham at about 10.30pm on April 21 last year when he was confronted by two men.
That confrontation resulted in the 21-year-old being shot and falling to the ground where he was stabbed.
Detective Chief Inspector Will Reynolds, who is leading the investigation said: "Ola left his home at 10.15pm that night. He was shot at 10.30pm. It would have taken him a maximum of two minutes to get to East Surrey Grove on his bike so where was he? Did he meet anyone en route? Those 15 minutes are critical. I need that information and am desperate for someone to come forward to tell me.