@John *********
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.