For retirement, he needs either 800k baht in Thai bank or 65k baht monthly income into Thai bank (you make it sound like needs both).
The other option is combination of deposit and income to make up 800k, so if he has 50k pension per month, that's 600k per year so he'll need to have 200k deposit to make up the shortfall
Obviousy exchange rate fluctuation could drop his 50k monthly to example 45k so he would need to be careful (suggest increase deposit for safety net)