It does sound as if he's protecting the mum and kittens, some cats will do this. I deffo think he needs to be caught and neutered, it doesn't sound as if he's been done by the way he's acting. How old are the kittens? Are they old enough to be taken away from mum or are they still very young? If they are old enough to be taken away, as this other cat obviously cares about the mum cat, it may be worth releasing them together when they have both been spayed and neutered.