You need to provide more detailed information here regarding your son's 325 trip to get meaningful advice.
What exactly was he doing abroad? Where were his immediate family members (since he was a minor, this means his parents) at the time? What kind of ties did he retain to the U.S. during that trip? Etc.
Without that specific information, in principle he has three options:
1) He could try to apply now under the provisions of 8 CFR 316.5(c) (1) (ii), the so-called "4 years plus one day rule". See the text of 8 CFR 316.5(c) (1) (ii) at:
http://law.justia.com/cfr/title08/8-1.0.1.3.68.0.1.5.html
There is a Sept 1993 INS letter, listed as an appendix of the USCIS Adjudicator's Field Manual, which expresses the opinion that the 4 years plus one day rule does apply to trips between 6 months and one year in duration (like your son's trip). See the text of the letter here:
http://www.uscis.gov/ilink/docView/AFM/HTML/AFM/0-0-0-1/0-0-0-26573/0-0-0-40527.html
Under this option he became eligible for naturalization on April 30, 2012.
If he does decide to use this option, I suggest that in part 2 of N-400 (basis of eligibility) he checks the box "D (Other)" and write there something like: 8 CFR 316.5(c) (1) (ii), 4 years plus one day rule.
Otherwise the IO may miss the possibility of applying 8 CFR 316.5(c) (1) (ii) to his application and treat it as a regular 5 year case.
2) If he had really strong ties to the U.S. during that trip (and that depends on the information you did not provide us), he may try to argue that that 325 days trip did not disrupt his continuous residency. In that case in part 2 of N-400 he'd check Box A. However, he'd need to have really good supporting evidence in that case.
Under this scenario he could also file N-400 now.
3) If he wants to be super-cautious, he could apply 5 years minus 90 days (or later) since his return from that trip, that is, apply in early February 2013 or later. In that case he would not need to prove continuous residency during that trip.