Ian is a uniquely good position here: the truth happens to be one of the best explanations he could possibly give. The fact that he quit his job to look after his sick mother, assuming that his former employer has positive things to say about him, is a solid reflection of his character and a sign that he was out of the job market not due to a lack of options, but a need to take care of a sick family member.
Ian should tell the truth, and explain the decision thoroughly to demonstrate the kind of person he is to the prospective employer, and what his priorities are.