Ann I'd probably drive myself nuts if I didn't try to explore every possibilty.
I think Mark is completely correct that he "should have" been buried in the National Cemetery and probably is. He probably "should have" been in the Roll Of Honor also. Since he died of disease at a post with his regiment present, his body "should have" been easily identified when buried. Things seemed to have gone wrong somehow at some point and what "should have" happened seems not to have.
I can't find any evidence his body was retrieved by the family, especially since they left his brother buried in St. Louis. If they got one they must have gotten the other. If they left one they most likely left the other, unless it was a widow who did the retrieving of her husband only maybe. He doesn't appear to be in any local grave where they lived at the time.
Most likely somehow his identification was accidentally lost during a reburial effort, and he ended up in an "Unknown" grave.
BUT, if it isn't inconvenient for you to look for him in your sources, I'd sure appreciate being able to cross off any other possibilities. I'd rest easier knowing you had at least looked. THANKS