Ayush can be Dev's illegitimate son. When Sona & he separated he dated many girls. Remember Sona used to taunt him that he knows a lot about girls after they got back together. Even when Dev was extremely jealous of Jatin, Sona said why can't you tolerate me with my friend, when you have been with so many girls in these 7 years. Do you think I don't know about it?
Dev couldn't love anybody, he didn't give Sona's place to anyone else, he immediately broke up with girls when they confessed their love for him all these are true but he spent some enjoyable time with many girls, right? So there is a possibility of Ayush being his illigimate child. That's why he felt that immediate connection with him which Sonakshi can't. Since the makers trying to copy 80's film Masoom, they might have taken the whole idea from it. There too Nasiruddin Shah did not have any idea that he has a son with another lady. When he got to know he brought him home & initially his wife couldn't gel with the boy. Dur dur rehti thi n all. In Kuch rang the difference is they said Ayush is Sona's son& the whole baby swapping thing which I think is made up & Dev is being blackmailed by someone. He is scared & hiding the truth from Sona.