Hi...
1. Try the VA. They have records that might amaze you. My father's VA file (different than the St. Louis records) even had an evacuation tag from when he was with an AAA outfit near Seattle! You can request the files as next of kin and have the file sent to a lcal VA office wehre you can read it and take notes. The file should show his duty stations throughout his service time as well as other information.
2. Look to the county clerk of the county in which he lived upon his return to the States. and seek out his Discharge papers. Often (not always) States gave a small bounty to vets who filed a copy of their discharge. If the County doesn't have it, then sometimes the State has it.If it is in Illinois, you will need to be next of kin also.
3. Once you ID his unit, then you can obtain copies of unit records from the National Archives (NARA on line) including monthly summaries, After Action Reports, message logs and other miscellaneous reports/documents. Not all units kept good records, and not all records made it to The National Archives...but well worth a shot. ALSO, if you have the unit, you MIGHT be able to find daily reports in St. Louis that were missed by the fire in '73.