It seems that he's not interested in the cat welfare angle, just in the money, therefore aim your argument at his pocket. Say something like, "Don't you find it expensive, feeding all those cats, and the vets bills when things go wrong with the birth? It must be really costly to have to take the pregnant cat for vet checks."
If he replies something like, "we don't really bother with all that, just let 'em get on with it," you've got him. The RSPCA have the power now to find out his address from his employer and to prosecute him on the grounds of neglect (he's not taking care of his queens).
People like him don't deserve cats!