The expiration date of USCIS forms has no relevance to you, the applicant. All you have to make sure is that the edition date (which is on the lower left corner of every page of the form) is one of the editions listed as currently being accepted on the USCIS page for the form. The latest available edition of any form is always accepted, regardless of its expiration date. For some forms, they may also accept several past editions of the form (some of which may have been expired for a long time). Conversely, in some cases a non-expired edition of the form may no longer be accepted, so you cannot rely on the edition being not "expired" to mean you can file it.
The webpage for N-600 says that currently only the 02/13/17 edition is accepted. If that is the edition of the form you have, then it is good to file. The form webpage section on editions will have a notice starting 60 days before they stop accepting a particular edition of a form, so if you check the webpage soon before you mail the form, you shouldn't risk your edition not being accepted.
By the way, N-600 can only be filed for someone who has already automatically become a US citizen, so if you are filing N-600 for your son, he is a US citizen, not "is eligible for US citizenship". And since he is already a US citizen, you can get a US passport for him directly at any time, without first filing N-600 for a Certificate of Citizenship. The same evidence that he is already a US citizen that you would file with an N-600 can be used as evidence of citizenship for him to get a US passport directly. A US passport is much cheaper and faster to get (costs $165 for over 16 or $135 for under 16, and takes weeks to process) than a Certificate of Citizenship (costs $1,170, and takes months to process).
