If you're really worried, then seeing a pead for reassurance might be a good idea.

It really sounds to me like he's doing wondefully and is just a big boy, as your nurse says.

All babies are so different in the way they grow. My DS was 4.3kg at birth, but grew very slowly and is quite a small, slight thing.
I know someone who's baby was born about 4.8kg and has always been massive. He's tall and heavy, like your boy. But perfectly healthy.