My nine year old nephew is so perfectly behaved....

I wasn't able to be a mum(sadly) so I don't know a lot about children. I am from a big family and, from what I can remember, we were a rowdy bunch...well behaved, but always talkative, etc. I helped in a pre-school (where boys were rowdy) and taught scripture for a few years. The majority of boys were talkative and difficult to control.
My nine year old nephew is so quiet but absolutely well-behaved. When I go to visit my SIL, he will sit in a chair quietly with a glass of milk and just sit and listen. After drinking the milk, he will politely wipe his mouth. He won't interrupt but will answer questions. eg,"How is school J?" "Good - thank-you"
Yesterday I was in the car with my SIL and her children and I was sitting next to him in the back seat. He sits perfectly straight(never slouches) walks on his toes, straight and tall. He sat quietly in the car -says nothing.
For a boy he is perfect. The little boys I know are rowdy, loud and always on the go.
He is the model child!! When he is with his other cousins, he plays quietly.

10/10 to his parents who have raised him and his siblings well.
But is this too abnormal for a young boy to be this good?

It's hard to tell from that whether there's anything there of concern. Maybe he is just that. Quiet and reserved.

It is somewhat unusual for a boy of his age but there's a whole range of 'normal' behaviour. The only thing that stands out to me is that you say he walks on his toes. Not a big issue on it's own but maybe a symptom of other issues. I guess it's not really for you to explore unless his parents are concerned and ask for help/opinion but I would want to know what he's like when you're not there, whether he plays well with his cousins (by well I don't mean being quiet and 'good', I mean being able to join with them appropriately, understand their games and follow their social cues)....

Have you commented on his behaviour? What does his Mum say?

I think every child is different. What you consider "perfect", I wouldn't consider "normal". But then, that's me. Some people like to raise their children to be quiet and reserved, I like my boys to be rowdy and loud. To each their own.

Every child is different. Nobody is "perfect".

He sounds a little unusual - if he's always that way.
perhaps he's shy? some kids like things to be "just so" he may have those tendances.
If you feel something isn't right, then go with your feeling and perhaps try to spend more time with them? does he seem.relaxed and natural or on edge?
could be lots of explanations :)

Thanks for responding. I often say how well behaved he is and she just smiles and says, "yeah, he's a good boy".

At Christmastime I was over her place and noticed someone had given her a book(as a gift) about Autism/aspergers. I didn't say anything but had wondered if he was autistic. Not that it is any of my business. Just curious I guess.