Nice waves CSE, looked like a lot of fun! It must have been a blast with your son!
You mentioned needing to learn a backside turn....
Backside turns are actually very easy on SUP. I say that because SUPs are relatively big and the leverage available give backside an advantage over front side on SUP IMO.
When surfing backside, the mere act of placing your rear foot near the rail, transfers your weight to the rail through your heels and shifts your weight out over the tail because of the way your body position is with knees bent during a turn.
Add just a little drive with the back foot, and some upper body rodation and the board snaps around!
Commit over the rail and the board will follow!
Pic showing heels on the rail, body out over the rail.
Both pics are the same, I just rotated it in the 2nd pic to level the horizon so you could see the way my body weight applies force to the rail in the turn, simply by moving the heels near the rail.