I have to say I'm a bit 
at the assertion that he got 50/50 residency of his dd because "he's a narc lawyer." (And IMO the term Narc is bandied around here with far too much frequency). 50/50 is seen as the default starting point these days, it has nothing to do with who throws the much weight and everything to do with the child's right to an equal relationship with both parents. Presumably when you were together you didn't question this 50/50 arrangement?
Op you need to think about what is best for your DS in this situation, and try not to allow your hatred for your ex (with whom you were with for a number of years, had a child together, and who was a father figure to your ds while you were together) cloud your judgement.
You say your ds' dad died when he was very young. Does he remember his dad at all or is your ex the only father he has ever known? Because if so, and he A, has a relationship with his sister, and also a brother who is related to this man who is the only father he has ever known, then the relationship as far as he is concerned is very simple. This man has brought him up, you have been content with him being a father to your ds while you were together, and now that you're not you're wanting to remove that relationship because of your own feelings. Be honest, if the question of parental responsibility had come up at a time when you were together and happy, would you have pursued it?
At the end of the day we're only talking about something which at most will last for another seven years, by which time your ds will be an adult and be able to make his own decisions. In fact he will be able to decide when and where he goes once he turns fourteen, and the courts will take his views into account now anyway.
There was actually a thread on here a few months back from an op who was the step parent but whose dh's ex had died, and the dd lived with the ex's new partner who refused to allow her to go to her dad's and was going to go to court for PR - and they were told he had the same rights as the deceased mother because he had lived with the child for more than two years and therefore would be granted PR.
While I wouldn't be granting PR after a split, if the child had a parent-child relationship with that step parent and they viewed them as their parent on account of the fact they had raised them as their own in the absence of another parent, I would question whether perhaps it would be preferable to allow PR in this instance. Also, what would happen to your DS if something happened to you? Would he want to go and live with your ex and his brother, who would obviously go to live with him by default?
Granting PR in these instances means that both of your children can be seen as equals in terms of their relationships with both each other and the man who is your youngest ds' dad by blood and your eldest ds' dad by nurture.