Marcus lives in a dream world he has created to protect 17 year old Marcus. In this fantasy he is a superhero named 'Darkhorse' and he refuses to allow that character fantasy be 'repressed',
so,
All a person needs to do is point out any number of things he 'claims' and compare them to what he 'did' this Summer.
Then talk about all his lies.
Then he implodes by his own devices.