Literally everything he has to say on his experiences would be of the utmost interest to historians, amateurs or professionals. Even the most mundane of things, such as day to day life. Even the memoirs of a cook, or 'behind the lines' person is something we are always interested in hearing about!

Professional historians often have to rely heavily on first hand accounts, for better or worse, and human error, failing of the memories, and contradictions aside, the entire idea of a veteran's testimony is invaluable. It is a well that is rapidly drying up. The more first hand accounts we have, the more well equipped we are to study battles, debunk, or identify, myths, and add yet another voice to the clamor.

You should sit your grandfather down and just talk, perhaps with a pen and paper ready. Begin simply, avoid sensitive subjects at first. If a veteran is willing to talk, he will eventually speak of the more psychologically distressing incidents on his or her own accord. Ask him about his training, when he joined, the food, memorable moments, etc. The rest will come once he's relaxed.