The consensus in English-writing guides like the APA or Elements of Style is about staying true to a person's self-identified pronoun while preferring specificity over ambiguity. In a case where you don't know - "they" is perfectly acceptable. In a case where "they" is an explicit self-identity - it's the only correct option. In this case, where he literally calls himself "Mr", it's stylistically incorrect to choose a more ambiguous pronoun where "he" is the correct one for someone calling themselves "mr"
The consensus in English-writing guides like the APA or Elements of Style is about staying true to a person's self-identified pronoun while preferring specificity over ambiguity. In a case where you don't know - "they" is perfectly acceptable. In a case where "they" is an explicit self-identity - it's the only correct option. In this case, where he literally calls himself "Mr", it's stylistically incorrect to choose a more ambiguous pronoun where "he" is the correct one for someone calling themselves "mr"