This happens during the time when Naruto fights Kara's Delta. Kawaki takes the laser beam, which causes him to lose his arm in the process. He risks his own life for Naruto's sake. Afterward, Naruto's science team created a prosthetic arm for the young boy, but the attempt wasn't easy because of his unique DNA. Luckily, the Seventh Hokage had a spare as he lost his arm years ago fighting Sasuke. Naruto blessed the new arm with his chakra.