According to the South Dakota State Historical Society's Cemetery Records, he's still buried at Ft. Sully. Go to their web site, http://apps.sd.gov/applications/DT58Cemetery/
and enter his name and the county. The info will appear.
Underneath that there are a couple of other options about getting copies, etc. If you don't think this is correct, I'm sure they will want to hear from you. Successful Searching!