Accessibility for Shaking Hands, Parkinson's Disease

Hello Apple Community! My Dad has Parkinson's Disease and Shaking Hands make it difficult if not impossible to do simple tasks such as text messages, delete emails, etc. Even with using Siri and Text Dictation, there are two taps - and to try to make an edit if it doesn't dictate perfectly is a nightmare for shaking hands. Any advice on accessibility features of iPhones that could help. Anyone else with shaking hands and lack of fine motor skills find helpful new ways of operation?

Thank you!!