To derive US citizenship from you, he must be under 18 on the day that you are sworn in. If he turns 18 during your naturalization process, then he can not derive citizenship.
Nothing happens to his permanent residency. He remains a legal permanent resident if he turns 18 and can not derive US citizenship from you. He keeps his green card and can naturalize on his own under the 5 years rule. While his visa opportunity was derived from you, his legal permanent residency is not. His LPR status is his and independent of you.
