Hi and welcome to our forum.
If you cannot retrieve a copy of his discharge papers, please turn to either the local county building (where he lived) or get in touch with the local Veteran's Admin in his home state. Most soldiers filed a copy of their papers with the county or if they had any medical treatment with the VA, then they will also have copies. This is how I obtained a copy of my father's paperwork.
Here's the link for the VA - http://www2.va.gov/directory/guide/vetcenter_flsh.asp
Unfortunate as it may appear, many times things happened in the course of the war, making it impossible for injuries, etc., to be recorded and therefore impossible to make it possible to get the proper medals later in life. As in his case, you would have to find the proper documentation in order to proceed. What I would recommend is to try and obtain copies of the units history via NARA in Maryland. This is NOT the same establishment as St Louis. The Maryland facility contains archived unit histories.
http://www.archives.gov/dc-metro/college-park/researcher-info.html
You can either hire a research assistant or schedule a time to go there yourself. Complete info and instructions are listed on the link above.
Have you had time to look at the links and uploaded documents earlier in this posting? They will help you pull together a more precise timeline.
As I did for others, i will add your name to my 257th group and introduce you to them. I'm sure they will be able to assist you too.
Let me know if you have any other questions. Happy to try and help.
Proud Daughter of Walter (Monday) Poniedzialek
540th Engineer Combat Regiment, 2833rd Bn, H&S Co, 4th Platoon
There's "No Bridge Too Far"