As they grow, the males do get feisty. They have some extendable spines, sort of below and a little behind the gill plate. Those are not the soft material of their mustaches. Rather it is a material closer to teeth and they can gouge or scrap rivals.
I was lucky to have two females and a male. (Of course the male killed his rival.) Once they left me with two balls of amber eggs the same day. The male couldn't quit grinning.
With guppies we have already talked about moving the males to their own tank so they would cause less mischief. I have found that tanks of extra killies are remarkably peaceful. Their behavior is less competitive without the presence of females and perhaps, their pheromones.
An interesting experiment might be to introduce water to such an aquarium from the tank which held females of one of those species.
As the bristlenoses get bigger, it could best to remove the boys to a tank where they could be useful on algae patrol, but wouldn't get into fights.
By the way, your son's 4-incher isn't a baby. I have had some get twice that size, but that is probably a spawnable fish. There are a lot of species in Ancistrus and related genera.
You might have your son turn off the tank light and some of the room lights and just watch those two in his tank, to see what they are doing. How big is that aquarium by the way? There might be a space issue.
Catfish can make a lot of noise at night. The reasons are numerous. "I got it! I got that guppy!"
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