Such a person has limited options when it comes to resolving differences between himself and others and between different aspects of himself. The re-gaining of this most characteristic of human activities is often the task of the therapist. And so, as a therapist, I am often trying to impart a humorous angle on things. Hypnotic techniques are of course a useful aid. You can create funny pictures in a person's mind that much easier.
Physical relaxation is generally more favourable for humour.
And, in a good subject, the process of laughter can often be activated easily. Mother's face disappears. The baby's face grows solemn. The mother's face re-appears. The baby chortles with a special delight.