My grandfather fought in the trenches during WW-1 and as a result, had foot problems for the rest of his life. I truly regret not ever taking the time to actually sit down and talk to him about his life. But as a young kid, one never gives much thought to things like that........
My guess is that your grandfather suffered permanent nerve damage in his feet from trench foot, basically a slow-moving form of frostbite from his feet being cold and wet for weeks at a time.
Given the horrors of war and the usually reserved nature of many Veterans, It may well be that he might not have been too eager to speak at length about his experiences....I've found that to frequently be the case among WW2 Vets that I've had the honor to meet and your Grandfather may possibly have been similarly disinclined.