Hobby Lobby ....
Decal printing set comes with clear and white backed decal paper + printer software (if needed) ... just need an inkjet printer or access to a local print shop. I would choose the clear unless your base material (more on that later) clashes too much.
After that, go online and get some stuff to print .... perhaps the symbol for his battalion, division, or unit? Include name, rank, date promoted, rank symbol, whatever is desired... print a small range of sizes to see what fits best on your pens.
Now ... base material ... since you're applying a decal, it can be some fairly plain stuff like maple or poplar. Anything with a nice bright background for the decals to stand out on will work great! White or yellow plastics work also .... as does visibility green or orange. Sometimes using such bright colors on a battlefield would be questionable, it should mostly be hidden in a pocket, and as a soldier he should know when to keep it under wraps for safety.
Last, to keep that decal in place and increase the longevity and durability of the pen, applying a CA finish over the entire barrel + decals not only keeps the decals from peeling, it also keeps them from wearing off or possibly fading. Finish up with micromesh and polish OR take it up to only 8000 grit micromesh for a NON-REFLECTIVE matte finish.