A quick look at Hopper's compiled service record indicates he enlisted in January of 1862 at Shelbyville and was discharged in November of that same year. He bounced around between a few different Companies of the 11th MSM Cav and the 2nd MSM Cav. The reason for discharge was not given, He had relatively few service cards that gave little information. He was in Hannibal in June, but it didn't state why - could be duty related or sick. Was he 45 or older? They tended to discharge older men, but that really didn't happen until 1863. The 11th and 2nd MSM Cav were intensely pursuing Porter and his men in NE Missouri during this period and anything could have happened.
I suggest you get his Pension Files. It appears as if he had both Invalid (#345,872 - Cert 304878) and Widow (#705,207 - Cert 486073) pensions. You can order them though one of the on-line services like listed at the top of this page. If you get it, let me know what happened. I am researching the Kirksville Battle and events that happened around that timeframe in NE Missouri. You will also get a lot of genealogy info (where he was born, his wife(s) name, children's names, their birthdates, when he died, etc.)
asbury_craig AT hotmail.com
Is he from Shelby County? I live in Monroe County, just south of there.
Craig