Maybe More by DoriansKitten has a very adorable William. Reading it will make your heart fuzzy.
Summary: William Pratt is busy trying to blend in and survive his new life and school when Buffy Summers crashes into his life. He tries to convince himself that he is thrilled to simply be the friend of such a charismatic whirlwind, but could it be that she might, just maybe, be looking to him for more.Dampersandspoons' Misconceptions would fit your criteria as well, but she took the story down to re-work it. You'd have to message her and see if she'd e-mail it to you.
The Eternal Rocks Beneath by jnharrow ...for some reason it won't let me put in the hyperlink, but it's here on SR, so you should be able to find it without any trouble.
Summary: William Pratt is a bookish young man who has has recently moved from England to Sunnydale. Buffy Summers is failing English. What will William teach her? 
