If my grandfather who is 70 something years old has dementia and now has a subdural hematoma what are his chances of surviving?

Some studies have data on mortality depending on risk factors and presentation. The 30-day mortality was 17% who underwent surgery. With more severe injuries, as high as 60%. Risk factors for poor outcome include age greater than 60 with severe neurologic impairment, lower Glasgow Coma Scale (GSC) score, coma at presentation, and coagulopathy. There are also CT scan findings, but not well studied.

Since multiple factors must be considered in arriving at a prognosis, and none of these are provided in your post, it is not feasible to predict what may evolve. But the worst outcome, generically, would be a 60% mortality rate within one year.