Getting Our Cats To Accept A Newby

We have four adult cats, a female and three males, all neutered, lovingly cared for, up to date with innoculations etc., and living in a cat paradise in rural Buriram. A young male turned up, decided our place looked like a good spot and camped out for a few days. We took him in and had him neutered and got him his various shots and he's settled in just fine. Two of the males accept him. The problem is with the female and one of her sons who do not accept him and attack whenever an opportunity occurs. These are not "play" attacks - they're pretty serious and the newcomer has copped some wounds, but nothing too serious so far.

Do any of you cat fans have any suggestions as to how to modify the behaviour of the aggressive cats, or how to make the newcomer more acceptable to them (other than a new de-odourant, say). It becomes a fine balancing act to try and keep them from doing serious harm.

One thing I've noticed - when attacked, the newcomer (if he cannot get to his safe spot), goes on his back and pees at the attacking cat. I'm not sure if this is a deliberate defensive move or if he's literally had the p-ss scared out of him.

What do you think?