It is Esha's son.
Remember when the show leaped 20 years on the last show before it leaped Esha was talking to herself saying that her son is Canada? London something like that and that Abhay does not know and she wants to keep it like that.
I have a feeling he is coming back to take revenge on Abhay(thinking that Mahi is Kusum)and kills her and when he realizied that it was not Kusum, he is on a mission to kill her every chance he gets. Or it could be that maybe that something has happened to Esha(since she is not on the show anymore and I have an odd feeling that it has to do something with Aryaman who may have blackmailed her to get out of India or have something that could damage her)and her son has come back to take revenge. He probably looks just lilke ABHAY. I think that he may have turned out evil like his mother.
The other scenario is that Ajay Malia has a Mask like Abhays so people will suspect Abhay only. He probably went to kill kusum and killed mahi instead and is trying to finish the job.
The 3rd scenario could be that Abhay is really the killer. Remember when after arguing with Abhay and Kusum, Mahi ran to her room and Kkusum was gonna go but Abhay said No Kusum Let me go to her.
Anyways, let me know what you all think😃