You need to look up YouTube videos by Pond Boss. He has a PhD in biology, specializing in fisheries. I have a friend with a pond full of similarly stunted bass. His bass are small, his bluegill are too large for small bass to eat and they're competing for the same prey. We keep EVERYTHING we catch there and I've hardly caught a bass over 12" in 10 years there. Years ago, my PB came from there, but that's over now. The best balance would be to have a lot of smaller bluegill for the bass to supplement their diet of minnows. PB would recommend feeding your gills so they spawn more times in a summer. Apparently that happens when they're well-nourished.
Ideally, you and others could catch enough small bass to make a difference, but that's usually impossible with one person. You could pay someone to come shock up a lot of bass and and larger bream take them out of the food chain as predators. After that you might be able maintain the new balance. You could also introduce another food source that's too big for bream and not for bass, like threadfin shad. But I'd talk to someone in the know about that before you do it. Bass might just gobble them all up or they might die off if the pond gets too cold and they're stuck shallow. Maybe there are other types of forage you could stock.
I have another friend who has a pond with a concrete spillway (not a pipe) and during times of heavy rain, it overflows like crazy and every time that happens, his fish grow bigger for a while. For what that's worth.
Honestly, he probably doesn't need to target the big bass, if there are any. They aren't the problem. He needs to target the big bream and the small bass. They're in direct competition.