FIRST thing to do is visit your local Immigration Office, this as NOT every of the ove 80 offices are certified to perform the conversion from Visa Exempt Entry into the Non Immigrant O visa,
Better be good prepared than finding out you run out of time to arrange all things at the last moment.
a Work Permit is a booklet that allows you to work for the employer mentioned in this booklet and does not allow you to stay inside of Thailand. The stamp Admitted until or Extension of Stay permitted until xx/xx/xx/ stamo in your passport allows you to stay, but under the condition that the reason to get this stamp did not change.
As the work permit expired in May he is not allowed to work and his possible Exension of Stay expired also in May as the condition to get the stamp changed from employed in unemployed.
If that is the case, even you depening on his stamp lost your legal stay the moment the work permit and his stamps became invalid.
Please inform us about the stamps in the passport.
The requirement is minimum 2 months prior application only but some Immigration Officers will tell or ask you to keep the funds in your bank account during the 30 day under consideration period which you get first.
You have to go back to the same office on 1 Sep with your passport and this receipt to get the answer of your application.
Advice: Do not walk out the office if you do not understand what to do, just ask the officer who took all your documents after you ask him/her that you like to apply for a visa.