My son is getting married in spain in october, what does he need to do
to make it legal in the USA?.They are both citizens of the US.

Hi kvrc-ga -

As long as your son is legally married in Spain, his marriage will be
recognized in the United States.  From the US Department of State,
Bureau of Consular Affairs:

"In general, legal and valid marriages abroad are also legally valid
in the United States. Should the parties contracting marriage have any
questions about the validity of a marriage performed abroad, they
should contact the Attorney General of the state or states of domicile
of the parties."

http://travel.state.gov/law/citizenship/citizenship_763.html 

The second sentence in the above quote should not be a cause for
alarm.  It appears to be language inserted in recognition of the fact
that States regulate marriage in the US.

Another thing to note are the worlds "legal and valid."  Thus the rest
of the US Embassy directive on marrying in Spain is important.

I am a US citizen who was married in Italy. The legality of my
marriage has never been questioned in the USA--or anywhere else for
that matter.

Google search terms: recognition marriages Spain US
://www.google.com/search?q=recognition+marriages+Spain+US&start=0&ie=utf-8&oe=utf-8&client=firefox-a&rls=org.mozilla:en-US:official

Best of luck to your son and his bride.  

alanna-ga

Marriage will be valid in US, however, to document it they need to have
the marriage certificte either superlegalized or having an apostille.
Apostille is easier and should be requested in Spain. 
see:

http://en.wikipedia.org/wiki/Apostille