You will have to consider how his head is getting bigger over time...if he's on the same percentile that he was, then he's fine. It looks like he's growing, so I don't think it's much to worry about. Around 6 months there is usually a growth spurt, so he does need to eat more...and he will need to start solids if you haven't already...he should be ready for that by now.

Growth rates may slow as they grow older too.

The fontanel isn't going to close until he is about a year.