What application I need to file if I am a u.s citizen and I want my son to become a u.s citizen he is under age and his a u.s resident..

How did your son get here? Is he residing in the US legally?

If so, according to the United States' Citizen and Immigration site, he should automatically be a citizen. Their criteria is as follows:
1) At least one parent must be a US Citizen
2) Child must be under 18 years of age
3) Child must be residing in the United States in legal/physical custody of the US citizen parent based on lawful entry or permanent residence.
(See here: USCIS - Biological or Adopted Children Residing in the United States (http://www.uscis.gov/portal/site/uscis/menuitem.eb1d4c2a3e5b9ac89243c6a7543f6d1a/?vgnextoid=d4c3a3ac86aa3210VgnVCM100000b92ca60aRCR D&vgnextchannel=d4c3a3ac86aa3210VgnVCM100000b92ca60a RCRD))

Anyway, if that criteria is met, you need to fill out this form: USCIS - N-600, Application for Certificate of Citizenship (http://www.uscis.gov/portal/site/uscis/menuitem.5af9bb95919f35e66f614176543f6d1a/?vgnextoid=a936cac09aa5d010VgnVCM10000048f3d6a1RCR D&vgnextchannel=db029c7755cb9010VgnVCM10000045f3d6a1 RCRD)

If you have any question about the criteria, I'd give them a call to confirm (1-800-375-5283)
3)