I'm having trouble with this because the answers don't come easily.
The only John LeGrand I can find in the 1860 census in that region of Missouri is a 61-year-old farmer by that name born in NC and living in Liberty Township, southeast St. Francois County. He was married to 51-year-old, KY-born Sophia LeGrand, and their post office was at Iron Mountain. Their children were born in Kentucky, Indiana, and the youngest was born in MO. In the same township lived 40-year-old teamster born in KY Levi LeGrand and family, and in another household of that township lived 25-year-old collier William LeGrand born in Indiana. Is this the family you seek, and is this the John LeGrand you seek?
The reason I ask is that the online MO State Archives lists a military service record for a Private John Legrand who ENROLLED 26 July 1862 at Ironton, Iron County, in Captain Smith G. Breckenridge's Company I, of 32nd Enrolled Missouri Militia Regiment. The only other thing mentioned on Legrand's service record is that he was "not mustered July 30, 1862." Let me repeat that. Legrand was NOT mustered at that time, and this is the only record I can see for him. There are other military records for John LeGrands, but they are from other parts of Missouri. The exact same is true for Private William Legrand. He also enrolled the same day at Ironton in the same company and was NOT mustered later that month.
I have a couple of theories about what happened to these two LeGrand men of Liberty Township, southeast St. Francois County, MO, and why they were not mustered into service into the EMM when the program began in July/August 1862.
One theory I have is that these men of southeast St. Francois County were too far from the rest of their company to be called to duty. They lived in southeast St. Francois County while all the officers of Company I except one lived in southeast Washington County, some miles away. I looked up all the company officers in the 1860 MO Census Index and only Second Lieutenant John Weddle of Company I also lived in Liberty Township, St. Francois County. The rest including Captain Breckenridge lived in southeast Washington County. You see, the EMM units were originated in summer 1862 as a sort of local "grass roots" militia intended to remain at home in civilian pursuits mostly unless activated to meet a local emergency. The 32nd EMM regiment was mostly a Washington County and St. Francois County regiment. Weak as it sounds, that is one of my theories.
My other theory is more sinister. State law required ALL able-bodied Missouri men to enroll in their local EMM unit that summer, and about 50,000 men in all were accepted or mustered into service in the EMM. The remainder were not accepted into the EMM for one reason or another, but many, many of those rejected were turned away because they were considered southern in sympathy by their neighbors. The Union military in Missouri was not about to place southerners into the Union militia for obvious reasons. I first thought the EMM failed to muster in John LeGrand because of his age, but when I saw that they rejected 25-year-old William, too, well....I had to wonder.
Well, this meager bit is what I found and those are my weak theories about why the EMM may have failed to muster in both those LeGrand men. Most men of that region who were killed not in the military in 1864 (as you indicated your John LeGrand was killed) were slain because somebody considered them on the wrong side. The war was particularly bitter in southeast MO.
I would like to know how this turns out if you discover more. I would like to know if I came close to the truth. I wish you good hunting.