You are actually part of the "host team". I can imagine how sad your mother-in-law is if the great-grand-son is not there. Try to stand on her side to think about the situation. On the other hand, I also understand your worries because your son has such a nightmare. Try to let your son appear during the peak period, then ask the nanny to bring him home straight after.
I was 8 months pregnant when my por por passed away. My mother-in-law has suggested that I don't go to the funeral. I explained to her besides my family, your son is also the host and has to be there. I just ignored her. Por por is somebody that's very very close.