I really wouldn't worry about what size he is. Tell your cousin that everyone grows at a different rate, and that he will grow also. Besides, the size of a man's penis doesn't have anything to do with how well he loves and treats his partner. As I have found, that's the most important thing there is. You tell him to stop worrying abou the size of his penis and concentrate on learning how to love his partner, both of them will be much happier in the long run.
You guys will continue to grow in height and weight and everything else.. It is just puberty kicking in...