I find him as a private in Co E, 1st Regiment, Missouri Cavalry.

He is in Platte Co in 1950 census:
Name: Reubin Justus
Gender: Male
Race: White
Age: 13
Birth Year: abt 1837
Birthplace: Missouri
Home in 1850: Green, Platte, Missouri, USA
Line Number: 15
Dwelling Number: 237
Family Number: 237
Household Members Age
James Bird
37
Charlotte Bird
42
Malinda Bird
7
Spencer Bird
6
Westerly Justus
19
Emory Justus
17
Elias Justus
15
Reubin Justus 13
Elijah Justus
12
John Justus
10