I have departed Thailand with an Under Consideration stamp for a Thai Wife visa extension (and re-entry permit) and then returned to go to immigration on the required date ... no problem.
If he is an American and is using Social Security as his pension source, then he can log-on to the Social Security website at ssa dot gov, and download his benefits letter.
If he is an American using a pension source from a company, he should be able to contact the company and have them send him a benefits letter or statement. Many companies have a benefits website where he can download the letter/statement.
If he is an American using a pseudo-pension, as in transferring savings or investment funds, then he doesn't truly have a pension.
According to the Thailand Pass conditions it depends on which of the three schemes you use to enter Thailand ...
The Exemption from Quarantine option (aka Test and Go) requires: Travel from a port of departure in eligible country / territory and have stayed in eligible countries / territories for at least 21 consecutive days. You are required to stay 1 day in an approved SHA Extra+ or AQ hotel upon reentry to wait for RT-PCR test results.
The other two schemes do not have a requirement to stay out of country for a minimum number of days but capture you in a Sandbox for 7 days or Alternative Quarantine for 10 days upon reentry.
There are other conditions for all three schemes ...
I know these things ... I recommended going to her local office for a definition of the required documentation, noted the requirements vary office to office, and referenced my post as an example ...
I have clarified my comment and will stop contributing to this page ...
I would recommend as step 1 you go to the local Thai Immigration Office and ask for a list of the documentation you'll need to submit with your visa application when you have a Thai husband ...
As an example you could look at my recent post on what I submitted at the Chiang Mai office for my 'Thai Wife' visa ... the required documentation is not the same for a 'Thai Husband' and varies office-to-office ...
Note ... In our case, in Chiang Mai, change from Miss to Misses and change from maiden surname to husband's surname were two different sets of documents.