Many cats are sadistic bullies - so you are not on your own being a slave to such a monster.
Maybe your neighbour could try frightening your wee boy (water works) so that he won't venture into the garden next door. If the victim cat has any sense then it will give your garden a wide berth. Maybe victim cat could try some self-defence classes. :-)
My two are siblings and they squabble all the time - however they seldom get into a real battle - but it has happened. My wee boy gets beaten up by his sister - he hasn't got the sense to run for cover.