The dad comes, he didn't know he had a child. The dad is not a vampire or the cause for the child drinking blood. The dad was working as a pastries guy somewhere else and was only visiting. The s-mom suggests the dad should take the child back, she is sad about it. At the airport the s-mom cries and leaves the airport in a run. It is then revealed after a time skip of 6 years the child stayed with the s-mom. The child is in school and in the last chapter when they are eating cake celebrating the kids birthday, the s-mom let's the kid suck her arms blood like always and it turns out the blood tastes weird for the kid. The kid then tries cake and it turns out he can taste it. It isn't explained why he was born with the blood sucking side effect, we just gotta assume it goes away after some years have passed.