First we’d like to say that your feelings are important!
Nobody is allowed to come near you if you don’t like it.
But sometimes it’s difficult to say “no”.
Maybe because you like the person.
Maybe because the others in your group like the person.
Maybe he hasn’t noticed that you find it uncomfortable.
You can say to him:
You’re coming too close to me.
Can you please stop?
I don’t like it.
He must respect that.
You can also look for help.
Perhaps a male or female friend could be there when you talk to him.
If he doesn’t accept your “no”, look for help from adults you trust.
Or go to an advice centre.
Here are some advice centres: https://www.hilfeportal-missbrauch.de/nc/adressen/hilfe-in-ihrer-naehe/kartensuche.html?tx_nxshelpdesk_helpdesk%5binstitutionType%5d=21