I mean the kid is not a part of his life and family. He's not in touch with him. So what's really wrong if he didn't tell Rashami about it? It's not necessary for couples to tell everything about their family. Maybe Rashami too wasn't interested in knowing about his family.
The kid is part of Arhaan's past, someone Arhaan doesn't care about in his present life. Was it really necessary for Arhaan to tell Rashami about a mistake he made in the past? The kid is 13 years old, that means it has been more than a decade. It's not that he was going to marry Rashami without telling her. They were just in a relationship.