"So that's an Army outfit, right?"
Correct.
It's also a very interesting and historical photo in that the Howitzers have the old wooden spoked wheels. Most were outfitted with solid steel wheels.
You might casually ask him what unit he was with. Division, Regiment, etc. Write it down and start researching. You have the fact that he was in New Guinea. It's just a matter of tracking down which Pack Howitzer Field Artillery units were in New Guinea. You may be surprised at what you can dig up when you have a few prime facts.
Check this site:
http://www.ibiblio.org/hyperwar/USA/...nea/index.html
Ron