a visa is tied to a passport number though. My passport number is clearly mentioned on the visa so I still think this can be moved to another passport. That way you don't have to travel with several passports either.
I mean it is probably better to get a new passport but usually 6 months is enough to get the visa. But you will need to move the visa to the new passport when the time comes. That happens in the local immigration office.
No. I applied there In April and only had the amount of 500k for about 2 -3 weeks. I did provided 3 months statements which showed a lowest balance of around 150k. My statements did also show normal usage of income/expense.
Yes for sure, he needs the sponsor letter from his father. Access to the bank account of the sponsor is not a requirement so that letter from the bank he has can't hurt his application but won't be enough either.
The following is a copy of the financial requirements for DTV:
4. Financial evidence: amount of no less than 500,000 THB OR equivalent amount in foreign currency e.g. bank statements OR sponsorship letter covering living expenses.