He might find out through Buaji, hearing a conversation between the Dubey's, or even hearing Aarti talk about it to someone. I think he'll be really hurt and angry in the beginning, but he might try to hide it for the sake of their baby.
Or...
When Aarti finds out about Ansh donating his marrow, she'll be in the hospital and faint. Prashant will sign his name once again as Aarti's husband when filling out her admission papers because the doctor will say they need to start surgery ASAP. Yash will come to know through this and then he'll be so furious with Prashant, the Dubey's and even Aarti.
When Aarti wakes up, she might be traumatized because she lost her baby, or she might be happy that her baby is safe. If it's the latter, then Yash will surely stress her out and she might end up losing the child. If it's the former, then Yash might be a little calm around her because he'll feel hurt after seeing her in so much pain.
What are your thoughts?