The word "infantry" refers to a military service men trained to fight on foot. Your grandfather would have been assigned, after boot camp, to a Divison, then a Regiment, then a Brigade, then a Battalion, then a Compamy, then a Platoon, then a Section, then a Squad. There are Heavy Brigades, Infanrty Brigades, Aviation Brigades.
If his discharge records were destroyed in the NARA 1973 fire then contact the Dept of Veteran Affairs in the state he lived in right after his discharge (and any other state/county he moved to later) to see if he filed for any GII. Bill Benefits due him from that state and the Federal Govt. He would have given a copy of his DD214 the the local county VA office.
Also some local newspapers reported on the comings and goings of local men who enlisted from their area. There might be a "story" about him. Also if he graduated high school, see if there are any class reunions still being held. Contact the committe for help. Ask that a notice be posted requesting attendees memories of your grandfather be sent to you or contact you.