If a boy joins a Bear den, may he go back and earn the Tiger Cub and Wolf badges?

No. In the Cub Scout program, all boys in a den work toward the same badge. If a boy joins Cub Scouting as a 9-year-old, he must earn the Bobcat badge (all boys in Cub Scouting earn this badge), and then he will begin working on the Bear badge with his fellow Cub Scouts. He is not required to have earned the Tiger Cub or Wolf badges. Since those badges are for younger boys (7- and 8-year-olds), the requirements for those badges are below a 9-year-old's current level of ability, so "going back" to pick up those badges is not permitted.