“Just be yourself.”
Your son is starting at a new school and is worried about making friends with other students. You give him this advice about how to make friends.
Just be yourself.
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Just be yourself.
This is a commonly-used phrase in English. If you're "just being yourself", you are acting naturally and not worrying too much about what other people think of you.
This is often given as advice about how to make a good impression on other people.