Some unhelpful grumpy people on this thread.
12,500 baht at Maneerat, Soi post office
Will need Bangkok Bank account for using an agent so an extra cost for the agent to set that up, perhaps around 6k, there have been issues lately so need to check if OK currently.
That's basically it, plus address.
Edit: as he already has a retirement extension he can go himself to Bangkok Bank and open an account. 2nd road, top of soi 6 is a good branch