In the comics, Homelander doesn't have any offspring. His character is focused more on his own power and struggles rather than having a family with children.
Well, it could be an original character who was hidden away from Magneto for some reason. Maybe a human - mutant hybrid that Magneto didn't know about. This son could have powers that are not fully developed at first, and through his encounter with Magneto, he starts to discover and master his abilities. He could be a character who was raised by a family that was against Magneto's ideas, so there's a lot of conflict when they meet.
In this fanfic, Alucard meeting his son could be a very emotional event. His son could be a product of a long - forgotten relationship. At the meeting, Alucard might see a lot of himself in his son, both in appearance and in personality. They could have a tense conversation at first, with Alucard trying to figure out what his son wants from him. As time passes, they might develop a mutual respect. His son could teach Alucard things about humanity and family that he had long forgotten. And Alucard could in turn pass on his skills and knowledge to his son, leading to a new chapter in both of their lives.