Well, if he isn't already registered, I don't think he'll be able to apply at this point.
This is from uscis.gov, but it's from 2001. The more recent announcements only deal with extensions. just plug in TPS into the search window, and you'll pick up all the various announcements on the different TPS programs.
An alien who is a national of El Salvador (or in the case of a alien having no nationality, a person who last habitually resided in El Salvador) is eligible for TPS benefits if the alien (1) has been continuously physically present since the date of publication of the Federal Register notice; (2) continuously resided in the United States since February 13, 2001; (3) is admissible as an immigrant (some waivers of inadmissibility may be granted); and (4) registers for TPS within the time specified by the Attorney General—18 months from the date of publication in the Federal Register.