Jason has made the decision to stay on the island, instead of going back home with his friends. When you gain control, make your way to the exit of the temple, where you’ll be stopped by a drunken Dennis, who will tell of his time in America. After his story is finished, exit the temple and fast travel to Earnhardt’s mansion.
Once you arrive at the mansion, go and enter the cave where your friends are. You tell them that you are staying, and they are more than dissatisfied with your decision. You can talk to Keith and Oliver afterwards, although the conversation with Keith is short and pointless, while Oliver wants to stay with Dr. Earnhardt, but Jason says he has to tell his mom what happened to his brothers. Exit the cave to start the next mission.