@Stephen ********
He cannot use the pension for anything, because the embassy will not certify it. But during the entire first year he can transfer at least 40,000 baht (65,000 baht for retirement extension) to his Thai bank account from overseas. Then for next extension if he can show 12 monthly transfers, the immigration office may accept that.
He would need to check with them before starting down this process, as some immigration offices will not accept international transfers for marriage extensions. The only monthly income some accept is embassy certified income, or proof of income from a work permitted job inside of Thailand.