Child very guarded at school

My 10yo is a sweet, polite, and thoughtful child. We moved countries three years ago and he's not yet "fit in" at school, atleast this is how it seems to me. He does have a friend group he plays with at breaktime but no one invites him for playdates or birthdays (apart from one) and neither does he want to either. He has said he wonders if he is uncool, and that he finds it hard to make friends, and although he doesn't complain and goes to school without a fuss, he seems very guarded (as if on his best behaviour) the minute he enters school. His younger sister is the opposite and makes friends so easily and has loads.

In contrast, at home he is super happy, comfortable and acts so silly and whacky! Which is why I see the difference. We have asked and it seems he is not being bullied either.

Should we be concerned? or just let him be? I guess I just wish he had more fun at school being himself but don't want to project onto him either.