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)

Anyway, if that criteria is met, you need to fill out this form: USCIS - N-600, Application for Certificate of Citizenship

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