Australia requires the flight number of a flight into Bangkok. You could put an intended flight. You can find a lot of posts on Australian DTVERS application process here
Anonymous participant Someone posted a similar post recently; it should be in the threads and maybe have useful advice. There is flexibility with approval so not impossible to get a dependent visa for your son.
Depends on embassy, London asks to see confirmed tickets but not with all applicants. Japan requests confirmed flights. Malaysia requires confirmed flights and 6 months booked accommodation,
Vietnam has approved with recent deposits. I got my DTV from Taiwan with a very recent deposit, i think it is still the same. They are responsive to emails for you to double check, Singapore i think you need to be a registered resident or citizen to apply for the DTV there. Jakarta is a good option.