I have had autonomic neuropathy for many years. I have slow stomach emptying from it and balance issues. The PT guy I went to explained that there are receptors in our legs that assist us in knowing where we are in space. That is how we automatically balance when standing and moving around. With neuropathy these receptors are damaged. I need my eyes to see my surroundings to be able to get around. Or be able to hang on to something.

You might want to check into some physical therapy. It might not be an option at this time though. As far as car rides go maybe just recline back and close his eyes. Or try a blindfold. When I get a really bad vertigo attack, I have to lay down and keep my eyes closed. After a number of hours it will start to improve. The motion of the car moving will make it more difficult. I also carry motion sickness meds with me. I have not had a really bad case to try them out though.

Wishing only good things for you and everyone dealing with cancer.