Dear
@Grace,
congrats that it worked out with the custody issue.
So now if you would like to move in the place of your boyfriend, he needs to get the consent of his landlord.
Once you have this you both go to the Lageso/ Sozialamt( the office that pays your rent) and get the permission to move in with your boyfriend.
Its quite easy but make sure your boyfriend goes with you and all papers are with you.
Best regards,
Nilab