My DD1 was similar weight-wise, she was born at 2.9kg and didn't hit 6kg until almost 7m - at 12m she was 7.6kg and even now she is light for her age although she is quite tall. She just has that physique and takes after DP. Are you or DH small builds?
We aren't small builds, but we aren't big either. My family is fairly tall, I'm the shortest at 5'8", and solid build; DH's family is prob no taller than 5'10" and pretty average weight.
Guess I'll just see how he goes, who knows it might all change when we start solids.

It's just hard to know your bub is okay when everyone says how cute and healthy a big chubby baby is, whereas everyone says "oh he's soo tiny / how premature was he / you need to feed him more" and look as if there is something wrong with him