Do you know what I have to submit at Frankfurt immigration? I intend to take a vacation with my son, who's half-Filipino, half-German. I don't know what kind of documents we need. Or is there a question or interview? Hope there's an answer. Thank you.
Not a very good answer since I can quote no official sources but it may be helpful and is too long for a comment.
I am British and my wife is originally Filipino but now a dual citizen. We have not made a trip quite yours but we have made some similar ones.
Our son was born in the Philippines but we moved to the UK while he was still a baby. I went ahead so my wife and son travelled alone. She had a Filipino passport and he had a UK one. They had no problems at either end. Of course, my wife needed a visa but that had already been arranged.
My wife and I took her sister's daughter from the Philippines to the UK. This was more complicated as neither of her parents were travelling. We needed a formal document from the parents giving their consent and this was checked carefully as we left the Philippines.
Later, when he was older but still a child, our son travelled to the Philippines with his cousin who was also still a child. He had both passports and she had Filipino only. They had no problems in either direction.
Many Filipino friends have travelled from the UK or France to the Philippines with their children but without the fathers. None has performed any special procedure or had a problem. They simply bought the tickets and got on the plane. I don't happen to know any Filipinos in Germany.
Your son will be entitled to dual citizenship but do you actually have both passports for him? That might make it easier as there will be no doubt that he may enter either country.
Since you will be travelling with him, I do not expect a problem. If you want to be very cautious then contact the Philippine embassy in Germany before leaving. For a lower degree of caution, get a letter from the father giving his consent. Bring copies of the father's ID e.g. passport and driving licence. Also bring your son's birth certificate. For extra caution get the consent letter witnessed by as important a person as you can manage.