Yes, look closely at the dates when he was a PVT in the 49th IL
Enlisted SEP 5, 1861 Waterloo IL for three years, discharged MAY 5, 1863
That dovetails with the ORs,
Reports of Cap. Stewart R. Tresilian, engineer Officer THIRD DIVISION, including operations since April 25. HDQRS. THIRD DIVISION, SEVENTEENTH ARMY CORPS, Camp in rear of Vicksburg MISS. June 1, 1863.
https://ehistory.osu.edu/books/official-records/037/0203
The Find A Grave calls him COL Tresilian - which is a full three grades over Captain. Without a photo of his headstone, hard to tell if that's someone's misinterpretation or not. He may have been Brevet promoted right after the war - who knows?
So if I were you, I'd have someone with FOLD3 access search the Federal Records for officers or engineers to see if some variation of Tresilian's name surfaces.
Then in the 3rd IL Cav (Consolidated - which might as well be another unit, when searching FOLD3 for tidbits)
mustered in February24th 1865 and mustered out June 3rd 1865