When Nadeko had the curse on her, she was almost going to be killed by it and it had only been a short time since she received the curse. Since the curse was removed, reversed, and was given to the boy, hasn't the curse had many months to kill him?
Because Jagirinawa tried to kill Nadeko when the ritual was performed, there are 2 snakes on her body. The ritual of removal can be done only for one snake, the ritual is not strong enough to remove 2. So Araragi forcefully removed the rest, but was not strong enough and was stopped by Kanbaru by letting it go. Nadeko only have the pain from the curse, the boy didn't die because he's not doing the removal ritual and have no two curse in his body. So if the curse returns back to the boy, he only gets the pain.