DTV with a soft power scheme such as Muay Thai. The colloquial "retirement" visa has nothing to do with retirement, it's criteria definition is 50 years of age or older.
What passport does he hold? As Jennifer noted you want to check the specific embassy website, not the generic e-visa application website. See if you can get some kind of residence certificate to show you have status in the UK.
you can't get any type of tourist class short term visa inside the country. The paperwork to switch to any other type would be time consuming and onerous, maybe if you had Thai family you could get 60 day visit extension. I can't think of anything else you might qualify for. A quick flight out and back is the easiest if your passport qualifies for visa exempt.