Some of the space on the iPad is taken up by the operating system and the built-in apps (and how a gig is calculated/specified), so he will have less than the 16 / 64 etc available to him - on a 16 gig iPad I think it's just under 13 gig
ebooks tend to be fairly small, most will be under 2 meg. Some books include video and audio in them which will increase their size, and there are textbooks available which are over 1 gig each, but 'ordinary' novels etc are generally under 2 meg.
Audiobooks also vary in size, depending upon the book (and whether it's bridged or unabridged). For the fiction 'Top Audiobooks' here in the UK one book is 928 meg, though none of the others in the top 10 is over 350 meg.
Note that audiobooks are a one-time only download from the iTunes store (so won't be able to redownload them for free), so if he buys any on the iPad he should copy them over to his computer's iTunes library via File > Devices > Transfer Purchases so that he can take a backup copy - or buy them on the computer and sync them to the iPad.
The size of an book or audiobook should be on its description page in the store
If he wants to have a number of audiobooks on the iPad then the 64 gig version will be better than the 16. If he wants to copy a fair number of photos from a computer to it then the 64 gig would also be better. The capacity of the iPad can't be increased, it's best to have too much space than not enough.