gcse ..english

I think they should definitely make reasonable adustments so that there is a level-playing field. Perhaps the same task should be set but with the question worded differently, avoiding the use of the word imagine? You could run through different imaginary possibilities with your son before the exams so that he is as prepared as possible.

Is this something the school could help with? Preview the question and give him some sort of experience to help visualise the imaginary person? My daughter wouldn't be able to answer that type of question but when she was in Primary, she won an award for a letter she wrote as a world war evacuee because before she wrote it, they'd spent a day dressed up as evacuees and had renacted leaving parents, getting on the train and then being allocated to homes in a village.  I think its the only piece of imaginery writing she's written with some degree of understanding.

I know that at university level these standpoints are not popular with students. They are used, especially in exams, to discourage people from brain-dumping everything they know (including a lot that's irrelevant), and to get students to argue a case in an applied context, rather than be merely descriptive.

Possibly this is asked in schools to prepare them for university, but it is broadly valid as it is to discourage purely descriptive facts for facts sake writing. Also employers complain bitterly about the writing styles of school leavers and graduates being too essayish and florid and not really useful to a business or scientific application.

It is also a pain for examiners and tutors as we have to vary the questions so they all start from a different premise - explain, discuss, elucidate with examples, debate etc. Questions which don't show enough variation can be thrown back at a tutor from the examiners, and you can get into quite a bit of trouble for not being good at setting premises.

So I'm afraid it is likely to remain a scourge of school pupils and undergraduates for the forseable future.

I know the problem - in that I had difficulty writing imaginary texts, especially personal letters and other things which teachers thought we ought to know how to do, and always gives me discomfort about the use of relational wording. Imagining I was an inanimate object was a whole lot easier (history of a penny was a personal best). I remember being mortified at having to write a thank you letter in class time as a piece of assessment. I couldn't handle the emotional/relational conventions required.

Imagine... type questions are used. They are similar to Discuss.... Explain... etc., which children need to be good at, but the problem may be the difference between actual and effect.

All he has to do is to take himself out of context and explain something, or give a narrative. The explanation is largely independent of the overriding concept, in that he could practice stock answers, or how to write such answers, and learn some conventions to go round it. 

Not all the marks should be tied up in imagining you are someone else, they usually have to include presentation, style, spelling, grammatical construction etc., so if he could just get round the conventions of non-self, he would probably still do alright on marks. But I can well understand if he is cringing at the thought.

Getting off the exercise depends what is actually specified in the curriculum. Is imagining you are someone else a specified learning outcome? I doubt it. You could talk to his teacher about doing a variant of the exercise.