WARNING: The following contains major spoilers for Boruto: Naruto Next Generations #44 by Masashi Kishimoto, Mikio Ikemoto and Ukyo Kodachi, available in English from Viz Media.
The Boruto manga has been just as much a Kawaki story as it has been about the son of Naruto. He basically became the new Sasuke -- in his case, Kawaki's a weapon of mass destruction engineered by Jigen's terrorist cell, Kara. Luckily, the Hokage of Konoha took him in and Kawaki became a brother-like figure to Boruto.
It's why he decided to accompany Boruto's Team 7 to rescue Naruto from Boro, as he saw Naruto as the father he never had. Upon successfully completing the mission, however, chapter #43 has revealed Kawaki may have a death sentence tied to Naruto's assistance in the past.
Kawaki lost an arm in his run-ins with Kara as he didn't want to go back to be Jigen's vessel. However, Naruto's science team engineered a prosthetic arm for the boy and Naruto used his chakra to bind it to Kawaki. This allowed Kawaki to be connected to the leader, which helped him track Naruto after the latter was kidnapped, but it also slightly augmented Kawaki as Naruto's chakra is more or less a massive power boost.
But after the fight with Boro expended most of his energy and damaged his body, the prognosis isn't good as Kawaki recovers. Naruto's science team reveals Kawaki's body is rejecting the healing process and any medicine they're attempting to administer. It seems the nanites in his body, as he was grown by Jigen's own science team, are not integrating with the meds, and it's posited that Kawaki's makeup has been altered due to interaction with Naruto's chakra. There are apparently long-lasting effects, and while in the interim Kawaki was boosted in battle, he was indeed paying a price on the inside.
His bodily functions are declining and he's experiencing muscular pain stemming from cell degeneration. He doesn't want painkillers, but as the doctors reveal, they don't even know if they'd work as Kawaki's body is so complex. He's not human and it's far beyond their understanding. In the past, Naruto's chakra has had healing abilities, so this is very ominous if it's actually degrading Kawaki's very being.
Seeing as the science team in Konoha also doesn't fully understand Naruto's chakra -- as it's tied to the Nine Tails, Kurama, inside him -- it just complicates matters. There's no simple diagnosis, and even if they think of a treatment, Konoha's doctors just don't know if it'll mess with Kawaki's body even more, especially as so little is known about his Karma mark.
Luckily for them, there might be a solution on hand with Amado arriving. He's defected from Kara, and as Jigen's science head, he's primed to come up with a solution. He oversaw Kawaki's creation and understands his biology better than anyone, so hopefully, he can augment it given its link to Naruto's chakra. He hasn't dealt with Naruto's life essence before, but as he's shown, he's quite the genius. If anyone can save the kid, it's Amado.