Just to say how sorry I was to hear you were bereaved
and then had this relative mess to clear up
I think you may be reduced to writing to each and every bank
The only other thing would be to ask the new tenants of where he was living to send on all his mail one of which would be a bank statement
or if he were employed ask the employer for the bank details they paid into.
Data protection is not an issue - remember that you are your sons rep on earth and can do anything in law he could ( same rule applies in OZ) .- Effectively in law he ( as you ) are demanding his own data