There's a great iPad app available for kids with visual disabilities. It's called Read2Go, and it will allow you to set the colors of the display (white on black is a common selection, but this app will allow you to select a variety of background and text colors. I personally love orange text on a black background, especially around Halloween!) You can also modify the font size (from 12 to 72 point, I think).
The app will also read the book aloud while highlighting each word being spoken. Many kids with learning disabilities benefit from seeing and hearing the text at the same time. The highlighting feature can be adjusted: you can turn off the audio portion and just have the highlight moving at your child's reading rate, if you wish. This highlighting feature seems to be very helpful for readers who have difficulty with finding/reading certain portions of the screen or staying on the same line.
The app is $19.99 for Bookshare members. Bookshare membership is only available to folks with "print disabilities", ie: visual, physical, or reading disabilities. While adults might need to pay a membership fee, students like your child are eligible for free membership. You can learn more at:
http://www.bookshare.org. There's also a demo video at
http://read2go.org/.
With an iPad, you'll also get the ability to reverse black-and-white in all apps, and you'll have access to the built-in VoiceOver screen reader, which will read most screens aloud, including books in the ibooks app, the Kindle app, etc. (I could be wrong about the Kindle app, I haven't checked this out myself yet.)