It's probably too late. If you want to try you'll need 2 more males and 2 females and put them all in a new tank not the one he's in now. If you add anything to
The current setup he'll see it as an intrusion on his territory and will defiantly attack & kill then.
In a new tank they'll all be on equalgku d territory wise but There will be fights and deaths, & No Guarantee he'll win them either ...
actually , he is in 4 ft tank , I move him in 5ft tank with two females , that's my plan , I know he kills everything in his present tank ( he already killed two sucker fishes )
That plan most likely will end badly . The shock of switching tanks might keep them timid a couple of days but once settled the most dominate female will kill her competition and then kill your male .