You probably not find anything more from ST Louis since most of the WW-I records were destroyed in the 1973 fire.
You have more information about this person than most of the persons requesting info.
I suggest that you search for the History of the 18th Infantry Regt. That will tell you what was going on and where the Regt was prior to that time. Best place is AHEC Carlisle PA. There you might also find rosters. Best to go there your self or pay a specific researcher to look for you.