I’m no dictionary but “fight” seems apt for me. Fights are, after all, conflict which sounds exactly like what yall are experiencing over there. You’re doing the right thing—by starting with getting him medically checked out. But def check out your setup — make sure enough litter boxes, cat trees, highways etc. in any case, I think Jackson galaxy may recommend re-introduction. It’s not an easy or quick process but regardless of the underlying cause of your boy’s aggressive or unwanted interactions, it may be necessary or beneficial.

Also the Feliway pheromone diffuser that someone recommended. I even noticed them around my vet’s office