The boy, who cannot be named due to his age, had admitted the manslaughter of Usaama, but was found not guilty of his murder.
He was sentenced at Isleworth Crown Court on Friday (September 9) to nine years in prison.
Ashkir Ali, brother of Usaama, said: "Today he got nine years, which is not enough for taking my brother's life. He should have been jailed for longer.
"Usaama was a funny brother. He was always happy and liked to joke about a lot, and we miss him terribly. I don't think justice has been served."
Usaama was stabbed in the chest following an argument with the defendant on Crown Street in Acton , on October 16, 2015.
Imran Ali, a close friend of Usaama, said in a statement in July: "He was a close friend, we saw each other a lot. He was a funny person and a happy person. I spoke to him on the phone the night before [his death].
"He seemed totally fine, he was himself. He was a popular guy, he will be missed a lot."
Detective Chief Inspector Rebecca Reeves, who led the investigation, said at the time: "Usaama's family are devastated by the loss of a son and brother.
"Why exactly this stabbing happened remains unclear. But what may have started out as a disagreement between two teenagers resulted in the death of a young man."