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?