I was tempted to say Vim, just out of habit... just kidding.
This seems like overkill, but you might be able to do it with LibreOffice write, or another word processor, depending on which one is available.
You can customize the toolbar to remove the buttons he won't need, and of course, you can choose the font and color. I think they all have a setting for autosave, or to save recovery information to allow reloading an unsaved file.
There may be simpler editors that will do what you want, and might be better if you don't have any of the open office applications, but if you already have them, it might be fairly easy to customize it.
I think customizing the existing editor would be easier than writing your own. :)
I meant to add that an advantage to the overkill method is that as he gets older and learns more, you can expose new features as he needs them.