If he does wind up being the son of Edmund and Cornelia, then VMI has him listed a New Market Cadet, Private, Co. C. so at least you'd have that much on his military record.
http://archivesweb.vmi.edu/rosters/record.php?ID=2978
A note at the bottom of the VMI historical rosters database says "For further information about an ancestor listed in the database, please
contact the VMI Archives. We can check the complete record, let you know if other information available, and provide copies of original documents. "
I wasn't able to find him in the Washington Artillery rosters. I have no idea how complete those records are--it looked in far better shape than the records for the one Conferate I've looked into in my family.
Also, there's a collection of papers and photos of Elizabeth Pendleton Coles (daughter of Colonel Edmund Pendleton and Cornelia Morgan) that have been archived at University of North Carolina. Some (all?) of the documents have been digitized, but not the photos. If one exists, a picture of the son in uniform could be helpful.
http://www2.lib.unc.edu/mss/inv/c/Coles,Elizabeth_Pendleton.html#folder_1#2
There are no letters or papers from 1870. There was one letter from Col. Edmund Pendleton to his wife dated April 6, 1865. (Seriously, why can't I ever find stuff like this for my own families?!)
In the letter he writes:
April 6, 1865
My Dear Wife:
The whole army of Genl Lee is now within ten miles Of Farmville. After a seven battle at Petersburg on Sunday 2d Apl. in which our lines were broken + our communications cut, we have been compelled to fall back to a more defensible line. I am sorry to think that this implies the abandonment of the greater part of V
a and it is not unlikely you + my dear children will be left in the Enemy's hands - tho' I hope not. I shall use my ut-most
? to come see you. Our Army is not whipped - indeed it is strong + ready to fight again to-day. I have seen ? several times on the March. He was in the fight and his guns he tells me fired 160 shots. He escaped untouched. I write this hasty note on the March and will endeavor to write more - fully in two or three days. Love to all.
Very affectionately,
E.P.
I'm uploading the letter here, and you can check on the line
"I have seen ? several times on the March. He was in the fight and his guns he tells me fired 160 shots." The ? is a name that isn't clear, but it could conceivably be Willie. Would be nice if it were a slam dunk, but I'm cutting the Colonel big time slack on the the handwriting seeing as how it was April 6, 1865 and he stopped to write his dear wife a letter.