He is certainly an honorary member of the military. His books were so popular, especially in the navy, that he has been able to tour nuclear subs, aircraft carriers and has even been on patrol with one as a civilian adviser/observer.
He has published numerous non-fiction books on various military units (Armored Cav, for one), their equipment, history and how they work. Rumor has it that after The Hunt for Red October, the White House had him debriefed to find out how he knew as much as he did, since some of the technical information in it was classified.
Tom Clancy wanted to be in the US military, but was unable to enlist. He was rejected from an ROTC program because of bad eyesight (he failed an eye exam).
Although he was never in the military, he has become quite familiar with military technology and the culture of the military. He has been able to do this to a great enough extent that he has a wide following in the military. It is said that he has contacts within the Pentagon who give him information that he can use in his books.
Though his novels have extensive military sophistication in terminology, usage, and characteristics, Clancy never was in the military. He wanted to serve in his ROTC at Loyola College but failed the eye exam. Tom Clancy's name is also now a brand with various authors now writing under the name of Tom Clancy. His extensive familiarity with the military gives his novels accuracy. His use of military terminology and his accurate descriptions of the use of military technology make him one of America's foremost literary authors in the genre of military fiction.