I think more background (and video) would be useful, but here is a first-pass guess…
I’m assuming that your boy is a right-handed pitcher and “throwing to the left” means from his perspective on the mound…so, he is throwing outside to right-handed hitters. Is he also throwing to the outside edge of the plate to the few left-handed hitters that he sees at this age?
In my experience, very young pitchers may have some hesitancy to come into the strike zone, especially against same-sided batters, because they often have a completely natural aversion to hitting the batter. The more empathetic your son is to the feelings of others, the more likely this is at least part of the story. Kids at 8 yo are often playing against their friends…even more reason for some hesitancy about hitting the batter.
I headed off this potential issue early with my son by having honest discussions about it…allow him to acknowledge that he doesn’t want to hit anyone on purpose–that’s not soft–it’s human nature, certainly at that age.
However, it is also very important to help him understand that it is completely the batter’s responsibility to get out of the way if a pitch does accidentally come too far inside. No matter how hard he thinks he throws at 8 yo, let him know that no batter is likely to be badly hurt from a pitch at this age…but, again, he needs to know that when a batter does get hit occasionally it is actually the batter’s fault, not the pitcher’s.