Since 'his' is implied but not required, I understand that from the English the Latin translation ignores it. But we do the reverse here, so nothing forbids to use 'a' or 'the' instead of his. This is the first time I see something implicit which require such level of interpretation. For me, it should be considered as a difficulty in itself, and thus require a specific level (where each topic would start to use implicit stuff) or a specific topic on its own. Not appear just for this single sentence. Especially if it is common in Latin, as mentionned above.