OP’s son could simply say that his mom is disabled, without disclosing the specifics of her condition. The focus needs to be on how this affects his day-to-day activities. There would be no implication that he is using her misfortune – the evaluator is asked to consider if there are significant obstacles, and how significantly they’ve affected the applicant’s opportunities to engage in other extra-curricular and academic activities. This is not different than the way the evaluator considers a student who must work after school and weekends on the family farm or in the family restaurant, and who, as a result, is less able to participate in other activities due to family responsibilities.