My in laws are going to Indiana (we live in PA) to my MIL's mom's for Thanksgiving. She is at the end of her battle with lymphoma and it will probably be the last time the extended family sees her, and her last holiday to celebrate period. My son's father didn't even ask our son if he wanted to go but it wasn't even a question that the other grandchild gets to go. My son has never met her, I haven't even personally met her, but she always sends him money for birthday and Christmas so I thought it would be a great experience for him to go and spend time with his part of the family he's never met.
I'm not sure how to tell him why he can't go. Using her illness as an "excuse" is unacceptable because my own mother, who he loves dearly, has been fighting cancer for 12 years. Plus it just wouldn't make sense why he is the only one not going. Not enough room in the car isn't valid because they're taking 2 cars and I would love for him to go so I can't lie and say I chose for him not to go.
I hate lying to my children but the real reason is probably that his father simply doesn't want to deal with him by himself that long, which I can't tell him that either.
Any advice on how I can tell him without lying, but not upsetting him either?

