he's supposedly unable to travel due to covid hence being on said extension so no he won't be able to get a re-entry permit for it. Nothing to stop him from trying though.
There's nothing stopping him from leaving the country but he'll need to go through all the hoops to get back into Thailand, including obtaining a visa should he need one to re-enter.
If you're doing a self transfer, which per my understanding is collecting your luggage, going through immigration and then checking in to your next flight, then you're going to be entering Thailand.
As such, you might need a visa depending upon your nationality as well as a Thai Pass.