He already went to Lak Si and they asked him only for any proof of financial support for the wife or perhaps his monthly income ie employment letter or bank statement. He should be ok to go and try again tomorrow.
It is a general requirement that the Marriage affirmation states your occupation and your income whether monthly or yearly. However UK and Australian Embassies changed their format and occupation and/or income are no longer stated in the affirmation. So the district office will now require you to show proof of income and/or occupation. Obviously some districts won’t ask these information and some will do. So please check which district to go to.
Maybe your marriage affirmation has the amount of your income in it. The current UK and Australian marriage affirmation does not state the monthly/annual income anymore. The embassy however provide you a letter explaining why.
It is basic requirement that a Male party needs to show proof of support for the female party. If the Male party is Thai, he also needs to show that proof.