I just get a feeling that Arjun and Ovi did not consummate, and Arjun is confused how the baby looks like him. Maybe even Ovi does not know that she did not consummate with Arjun as when she was drunk she ended up with someone else (Romil maybe?) thinking it's Arjun. Then Arjun pretended like he and Ovi consummated and much later when Ovi got pregnant he planned to hide the truth from her and give the baby his name.