I've got him as 4th Srgt, Co E, 15 MO Cav (Union), enlisted 1st of December 1863 to Co E 7th Regt, PEMM. Transfer to 15th MO Cav. Died Oct 19, 1864, in hospital Mt Vernon MO of Typhoid. He is listed as 22 yrs old, 5 ft 10 in tall fair complexion, black eyes dark hair and occupation as a farmer. He was a resident of Polk Twsp, Dade County MO at time of enlistment, furnished his own horse. Enlisted at Melville MO.

see:
https://www.fold3.com/image/228680188
https://www.fold3.com/image/228680220

His mother filed for a pension on him in 1890.
https://www.fold3.com/image/2642082

He is listed as born 1843 on July 1863 draft registration living in Dade County.
Find a grave is here https://www.findagrave.com/memorial/70906330/james-j.-montgomery?_gl=1*1lsu43q*_ga*Mjc5OTEwMjUuMTY1ODIzNDQyMA..*_ga_4QT8FMEX30*MTY1ODQ0MjM5NS4zLjEuMTY1ODQ0NDU0MS4w

Birth Date: 19 Feb 1842
Death Date: 18 Oct 1864
Cemetery: Weir Cemetery
Burial or Cremation Place: Greenfield, Dade County, Missouri, United States of America

1850 Census has him in Cedar County with parents:
Name: James Mounty
[Montgomery]
[Montgomery]
Gender: Male
Race: White
Age: 8
Birth Year: abt 1842
Birthplace: Missouri
Home in 1850: District 17, Cedar, Missouri, USA
Attended School: Yes
Line Number: 31
Dwelling Number: 413
Family Number: 413
Household Members Age
Jefferson D Mounty
40
Margaret S Mounty
28
James Mounty 8
Mary J Mounty
6
Elizabeth Mounty
5
Samuel R Mounty
2
Harriet C Mounty
2
Alby Jones Mounty
20

1860 census from Dade County:
Name: James J Montgomery
Age: 18
Birth Year: abt 1842
Gender: Male
Race: White
Birth Place: Missouri
Home in 1860: Center, Miami-Dade, Missouri
Post Office: Greenfield
Dwelling Number: 652
Family Number: 652
Attended School: Y
Household Members Age
Jefferson D Montgomery
49
Margaret S Montgomery
James J Montgomery 18
Mary J Montgomery
16
Samuel Montgomery
11
Catherine Montgomery
11
William J Montgomery
9
Finis E Montgomery
4
John Montgomery
1

Hope this fills in the blanks.--JJR