Sometimes just showing someone a video of their delivery lets them see things they don’t feel are happening. Show them someone else’s delivery that contains the desired posture and let them work on it.
At 8 years old, he’s not fully developed coordination, so it’s not life or death that he has this flaw, so he’s got some time to work it out.
Another thing you could try while he’s attempting to make the adjustment is to use rocker drills focusing on keeping is spine over his lower half.
If his back is arching toward the rear or 1B side, perhaps his center mass is not traveling over his front leg before he’s ready to throw. Often times someone who arches back has a center mass that is lower to the ground and further back toward the rubber than it should be.