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Perhaps contact the US Embassy and ask if they can assist in finding him.

A stage further from Stumpys suggestion.  "Definately" ask the US embassy, as you may well need proof from them later anyway.

Using an Annulment Based on "Presumed Death" is an easier way to get annulled, BUT, if you were to later apply for a visa to go to another country, especially the US,  the Embassy, during the visa interview, may want extensive evidence that the absent spouse was truly considered deceased.  If you have already checked with the US embassy, and have written documentation, then that should be sufficient for them, or other countries embassies.

The Family Code, specifically Articles 41 to 44, deals with the declaration of presumptive death for purposes of remarriage.

The Family Code of the Philippines lists Fraud as one the valid grounds for annulment:

If he used a fake name, I would assume that to be fraud of one kind.

"[3] Proof required that petitioner tried to look for the missing spouse

The court will however require the petitioner to present proof that he or she exerted earnest and diligent efforts to locate the whereabouts of the missing spouse. These efforts include asking the police or NBI for help in locating the missing spouse, using the media like newspapers or radio, etc. The court will deny the petition if the petitioner cannot present such evidence of earnest and diligent efforts to locate the missing spouse."
The Family Code provides for a period of four years (ordinary absence) for a person to be able to go to court and file a petition for declaration of presumptive death of the missing spouse.