I smell some discrepancies. How could he have been issued a Non-Imm-O visa based on marriage on 15th of June in his home country, entered Thailand after this date and only being issued an extension until 16th of May LESS than one year later?
you are qualified for visa-exempt entry. But you still must obtain the COE. You will have to apply online and follow their instructions. Need health insurance, ASQ hotel booking, flight details, and the PCR test to board the flight etc.
for the applications to the 1-year permit of stay based on being married to a Thai, he needs to fulfill the financial requirements, which are either a deposit of 400,000 THB on a Thai bank account or proof of monthly income of a minimum of 40,000 THB
19.04.21 is the earliest date when they expect you back for the application to the 1-year Extension of Stay Permit based on marriage to a Thai. That's 30 days before your admitted stay ends. They expect you to show the financial proof needed for the 1-year extension. Nothing said that you can't apply for the 60-day Covid9 extension, or the 60-days extension based on family visit. You can, but these 60 days are not the right way to obtain a long-term stay in Thailand. Only the 1-year EOS based on marriage is. What goes for the documents for the Covid19 extension , you can use the search function of this group to find it "covid-19 extension"