I agree with the above posters. He won't seek help unless he wants it. It is difficult to accept that you may have a condition like AS after living as a teen or adult for several years without much noticeable difficulty.
Personally I favour the website over the book idea. If someone gave me a book about a condition they thought I had (unless, as is noted, it is a parent or other close family member) I might feel uncomfortable, like I'd have to discuss their AS suspicions within the context of myself when I read it. But by all means, if you think it would be helpful for him to read about the experiences of a person DX'd with AS, give him a book