Positive self-talk to enable him to develop self-control ("I can handle this and work something out with Mom or Dad.")
Keep a diary of his moments of discontent, along with potential solutions when he has to face them again.
Parents can anticipate situations that make meltdowns more likely. Don't agree to an activity until you have a plan to avoid trouble spots. Say, "I know you won't want to leave your friend's house at the end of the visit, so what would help you leave on a happy note?"