The 'stiff' style was common amongst shows of the 40s and 50s, so he really fit in well with the genre of the time. Cooper and Cagney were also of the period and similarly stiff actors, but big names. Of course, Cooper and Cagney didn't serve in the military.
Totally agree, that style of acting was common for those times. Dragnet is a perfect example. Even some of the great actors of that time like Yul Brynner could come off as stiff, and hes a damn legend. Of course there were always the exceptions like Gene Kelley and Fred Astaire.