Thanks for bringing your question to JustAnswer.
I'm sorry to hear this has happened to him. If the dementia symptoms didn't start until he got to the hospital, it's possible they are a reaction to a pain medication. If that's the case, he should regain his mental abilities when the pain meds are stopped. If the dementia began before the fall, the prognosis is more likely that this is a permanent change.
Sometimes years after the initial diagnosis of Parkinson's Disease, the person can also develop dementia. If this happens, the deterioration looks very similar to Alzheimer's disease, and would be permanent. Here are some links that will provide education on dementia in Parkinson's: http://www.webmd.com/parkinsons-disease/news/20041213/tracking-dementia-in-parkinsons-disease