Pretty sure this is on the page for Fawkes, but you need to do the Finding the Garden of Eden quest. It's part of the main story, after your father is killed by the Enclave. Enter Vault 87 through Little Lamplight and work your way through the vault. Fawkes is imprisoned near the end. Talk to him and agree to release him. After you escape from Raven Rock he can become a companion provided you have good karma.