Thank you for reaching out to the Wefugees Community.
I agree with Pan 123, declaring the fatherhood ("Vaterschaft") and the shared custody ("geteiltes Sorgerecht") will be an important step for the future residence permit.
Regarding the family reunification visa application from his country of origin, I am afraid he will need to prove to the immigration office (Ausländerbehörde) that he is not able to travel back for the time being and he exceptionally needs to skip the "normal procedure". Depending on the immigration office, this process can be indeed difficult and hiring a lawyer might be helpful in this case.
Would you agree @manrhei-mbeon or @mbeon-caritas?
Please don't hesitate to get back to us if any further clarification is needed or you have another question.
All the best,