OP, you're thinking about this in the wrong way. You cannot manipulate your DS in this way, and you cannot force him to go to India if he doesn't wish to do so. Relationships need to be based on mutual respect, not guilt and obligation.
I get why you're upset. It must be incredibly sad for his grandparents, and obviously it would be difficult - perhaps impossible - for them to travel the other way. I do think family is important, and don't agree with the other posters who say that those ties are irrelevant. However, you cannot force a relationship.
Perhaps it's time for you to reflect on how you've done things over the years. Have you worked really hard to facilitate a good relationship between your DS and his extended family or have you just assumed that he will continue to see them out of duty? How many times did you take him when he was a child? And how did you ensure that trips to India were interesting and enjoyable for him? How have you nurtured the relationships with your family between visits? What did you do to try and prevent DS from getting ill while he was there?
We have family in India. I really hope that dd will continue to see them when she is older, but there will be no pressure on her to do so. It will be her choice. Our role, as I see it, is to facilitate the development of the kind of relationships with her family in India which will make her want to go and see them in the future.
Please don't try to manipulate your adult son. If you do, you will end up just pushing him away.