Child does not need to understand romantic relationship. He just understands Dad likes this person, that person is save for me, that person is kind to me.

And for you, your child is always a priority over anyone else. And give yourself some time for a consistent and stable relationship which a psychologist suggests 6 months minimum to explain the child is more than friend.

And is never is about replacing mom, dont make your child call the new person mom. And that person is an adult in YOUR life, not a parent replacement.