Dear
@Sab,
thank you for being such a vibrant part of the community and sharing your brothers situation with us.
As for your question, he can only do it from the embassy in Poland.
You might wanna process the part from the German employer at the Ausländerbehörde but he needs to get the visa applictaion at the embassy.
I hope this will help you, if there is more information needed fell free to use the platform.
Best regards,
Nilab