It really depends on your local field office. As @mam521 stated if her N400 is complete and she takes her oath prior to his next birthday, then he can apply for a U.S. passport using her naturalization certificate, and then file an N600 to get his own certificate of citizenship. If not, he will need to file a separate N400. There is really nothing you can do now other than get her N400 in as soon as possible unless you think it is better for him to follow the N400 process, then you can delay her’s until he turns 18.
Good Luck!