Oooh, he flew a Hughes OH-6A "Loach" (officially Cayuse)? Vietnam War scout helicopter ops are a favorite topic of mine.
The other posters are basically correct; a 'Nam helicopter crewman would have normally been issued an M1911A1 or possibly a S&W or Colt .38Spl revolver, depending on what the local armorers had on hand.
However, during actual operations in 'Nam, the supply situation was often very fluid and unpredictable. This, combined with an overall shortage of qualified helicopter crews and the "short timer" attitude of many warrant officer* crewmen, seems to have led to a fairly high tolerance (by U.S. Army standards) for personally owned sidearms. According to memoirs I've read, some crewmen carried various commercial Gov't Model or Commander variants since the ammo was usually available through standard supply channels. Believe it or not, Colt Pythons seem to have been a somewhat common status symbol.
OTOH I seriously doubt that a commander of a relief unit in Germany would have tolerated a crewman carrying a Python!
The basic 'Nam scout helicopter tactic for locating the VC was to draw their fire, usually by flying the helicopter very close to them while being shot at- NOT a mission for the faint of heart!
Since these guys had a very high chance of being shot down literally within a few dozen yards of a heavily armed enemy infantry unit, most crewmen also carried Colt CAR-15 aka XM-177 carbines in the aircraft, since these were much more useful in an actual firefight than a pistol would be. (The CAR-15 was a full auto short-barrel version of the M-16, and forerunner of today's so-called "Commando" AR-15 variants.) A Loach also usually carried a crew chief / door gunner with an M-60 machine gun in the rear seat. The M-60 was usually mounted with a bungee cord to the ceiling, primarily to allow more angles of fire while flying, but also to allow the gun to be cut loose easily if the crew was shot down and things got really hairy.
*Footnote: The warrant officer program allowed aircrew candidates to sign up for short fixed-duration deployments and skip the administrative and leadership training that a commissioned officer would undergo. Since a typical 'Nam era warrant officer did not intend to make military service into a lifetime career, he did not have the same incentive to do things "by the book" that a junior commissioned officer would have.