Ken wanted to play with boobs. He did have a female friend called Jessie who was developing faster than usual but he didn't have the courage to ask her to be his girlfriend.
He couldn't think of a good way to get a girlfriend and time alone with her from the book without changing things about his life he'd rather stay the same. He didn't want to mess with his family's minds, nor did he really want to force anyone to like him.
So he decided that the best thing was for everyone to have boobs of thier own so he could play with his own.
He flipped through the book and made changes in two places, on the page about human males he added that they have breasts and on the page about human female sexual attraction he made them just as interested in men's chests as men are in theirs. As he changed the page about men he noticed the book added on several extra paragraphs about pheramones and times and reasons a man will lactate, generally in normal men they only trigger when they have thier own child but there are other things that can cause it.
As he was reading the things the book had made up on it's own he felt his chest expand, his breasts grew until they were a bit larger than one would expect on someone his age, it seemed that he like his friend Jessie was an early bloomer in that area. As soon as his boobs stopped growing he felt a bra form under his shirt, it pulled snug around his chest.
He was intrigued by his changes, but an idea had come to him, he didn't need to make anyone his girlfriend, he could just look up how girls he liked felt about him in the book until he found one who would want to date him. He considered undoing the changed but decided he sort of liked it.