At first, all fans including me thought that it could well be some sort of senjutsu but now after chapter 25, I think that it’s just how his body is made. At the climax of Boruto Naruto, Next Generations chapter 25 a shocking detail was revealed. When Kawaki dealt with Garou he suddenly passed out.
We saw Katasuke and others observing Kawaki’s body. And Katasuke immediately said that his passed out because of overheating. He also added that Kawaki’s body is modified, that is he is a modified human being. Kawaki isn’t a synthetic human but a real human whose blood vessels and other internal organs seem to be modified with the help of technology. Mitsuki was keen on knowing more about Kawaki as he himself is a synthetic human.