William older brother of Joel Dennis was born 1825. Certainly not unheard of but getting a little "long of the tooth" to have been a very active cavalryman.
James younger brother of Joel Dennis born 1838, would have been prime age to be a combatant. As would Elijah and Alexander. Nicodemus would have been too young.
William is living with mom Jemmima, wife Martha, Children Guyon and Ann, along with brothers Elijah, John, James, Alexander, Nicodemus, in Wayne Township Bollinger Co. in 1860.
I'm working on this family tree, It appears that there a several Dennis families in Wayne County in 1850 all from North Carolina. Several other Dennis men appear in the civil war record from the area including Dunklin, Bollinger, Wayne, Stoddard counties.