I found the following info that appears to be about John M Riggs of the 8th La Infantry, Co H:

John Riggs 1860 US Census, Louisiana. Rapides Parish, PO Alexandria
NARA – M653
19 years of age – Born in Texas
Head of household was W H Riggs, Farmer – Born in NC
Either 7 or 8 siblings (one Female, 29, may be step mother)

John Riggs 1850 US Census, Louisiana, DeSoto Parish
NARA M432_231; Page: 185B; Image: 51
8 years of age – Born in Texas
Head of household was William H. Riggs – Born in NC
Mother Catherine , 22 years old from NC, and 2 siblings (Harriet T and Lucy A)

John Riggs 1870 US Census, Houston, Texas
Precinct 4, Houston, Texas; Roll M593_1592; Page: 278B; Image: 565; Family History Library Film: 553091.
29 years old – Born in Texas
Head of household was Wm H Riggs – born NC
Mother Catherine – 46 years old – Born in NC
8 siblings including Lucy

One www.ancestry.com record indicates that this same John M. Riggs was in West Virginia in 1880, eventually died in Berkley, WV in 1925. There are US census records for John M Riggs for 1880, 1890, 1900, 1910 and 1920 - Living in WV with wife Mary and 9 children.

Checked the TEXAS 1860 US Census and there was only one, a John W. Riggs, 21 years old, living in Corpus Christi

I don't know for sure if this is the same John M. Riggs, but it seems possible.

Don Parker