If your grandfather was a boy during WWI, he may have been too young to service in the army so I wonder what he was doing in a "foxhole." Try searching through WWI histories about the fighting on the Eastern Front. A good place to start would be the New York Public Library digital book collection. I think another place might be Google books.
During WWI the German army advanced into Russia and reached at least as far east as Pinsk on a straight line north and south of the city. Somewhere west of that line would be the town of Kamienczyk. You might be able to find where and when the German army passed through Kamienczyk on their way into Russia, and read about any engagements that took place against Russian troops. Assuming that you have the correct spelling for the name of the town, keep in mind also that it was not uncommon for the names of towns to have changed and that in the Kingdom of Poland at least some of the towns had Polish and Russian spellings of the names. If you can find a larger city close to Kamienczyk, for instance Warsaw, that is where you might be able to find more combat history.
One final thought: There was the Russo-Polish War of 1919-20. Communist Russia invaded the newly emancipated Poland, but were then driven back in defeat by the Polish army. Is there a possibility your family's story about your grandfather occurred during that conflict?