I bought an iPad for somebody who is paralysed; primarily as a book reader for its ability to enlarge the text- but also to surf the net, etc. with a family member or caregiver using the touchpad. I want to know if there is a voice command so he is able to turn pages himself, rather than needing to rely on anyone else.
No - without some external hardware that would somehow touch the screen for you, voice input is for Siri / VoiceOver and not general accessibility controls.
You might look to a program like Instapaper where you can scroll the view by tilting the device but iBooks doesn't have any accelerometer based controls or voice controls at present. It's not that it couldn't be added, just that it hasn't.
I am visually impaired and have been to multiple VA Blind centers, to make a long story short the VA is issueing apple products, including iOS products, not only to the visually impaired but also Vets who suffer from TBI, Spinal trauma... This came up this past Feb. If the individual has head movement a stylus can be attached to an extension and used with the mouth, not only does this allow the person to turn pages but also will allow them to open apps and move through app screens, and possibly the ability to use the home button.