This book was about a boy at high school who started to develop animal characteristics and abilities. I remember particular scenes from this book, like the boy in science class catches a fly as if he had been a cat, and another bit where he manages to climb a tree as if he were a squirrel.
I think he had a girl he liked. She may have been named Karen, and they did eventually get a few kisses. I think that as the novel went on the boy gained more and more animal traits and abilities to the point that the government got involved (or some shady bad guys). They wanted to use him to do something BAD.
I read it a long time ago (late 80s or early 90s).