Upvote:3
My first thought is to go with whatever works. If the ticket system allows for no last, or surname, then follow the passport, since that is your travel ID. If it requires a surname, but allows for no given name, do that. If it requires both a surname and a given name, put Mr. as the given (first) name, and your father's name as the surname.
My guess is, the system will require both a last name and a first name, so XXX YYY and Mr.
If you read through other questions here, you'll find this appears to be fairly common with certain countries. As long as his name, XXX YYY appears in the passport, the visa, and the ticket, it's very unlikely that he'll have problems at all.