What is the reason my client’s son talks about his imaginary grandma?

Unfortunately, this is something nefarious. It is an extraterrestrial visitation that has created this seemingly fictitious character. What he is seeing is a screen memory implanted in his mind to make the figure of the extraterrestrial manipulating him to be a human female, as that is a friendly image to him, and the purpose of the subterfuge, to give him reassurance so he would cooperate readily. They are seeking to observe him and are doing evaluations of his capabilities at intervals for research purposes and that is why they do not wish to perturb him as they are evaluating the psychological and behavioral responses needing to be measured in a calm state of mind for greatest accuracy.