i tried something similar with my dad and an uncle (cancers) who were ill and they never tried the option. it was a bit heart-breaking at first but then i realized that i was somewhat proselitizing [sp] for my own biases and preferences on them, who prefered the traditional doctor/medicine approach and did not trust cannabis. for having seen them how the health system treated them, I know who I'd rather trust if I ever came to a similar situation, but it was not my decision to make, and I'm not sure if cannabis could have truly helped them if they did not already have positive experiences with recreational cannabis like I did. i'm not familiar with how cannabis influences parkinsons treatment, but i think it's a small soothing for cancer and it could have messed up my uncle's relationship with his wife if he had gotten too stoned.

so my suggestion would simply be that you take one for the team, mention to them what benefits you think cannabis might bring your grand-father (back it up with a few sources or leads for further researching ) and that you would gladly support or assist him if he wanted to try it for himself (which is what I think you're implying, would you be able to get some for them on the black market if they requested it?)

be prepared for them to be offended at the suggestion, if so just move on and never mention it again. remember, it's his illness, not yours. hopefully your family will understand that you're well-intentionned.