Captured and turned into a guinea pig centuries before by Circe along with his crew, he is inadvertently set free by Annabeth and he is left marooned on the island when Percy and Annabeth steal the Queen Anne's Revenge to escape.
Centuries ago, Circe transformed him and his crew into guinea pigs. They were freed by Annabeth, resulting in their being stranded on an island when Percy and Annabeth stole the Queen Anne's Revenge to escape.