DTV is only good for 6 months stretches, retirement is indefinite stay. DTV is relatively new. Expect some changes in the coming years as they evaluate how is being used. It is still a tourist visa.
nor does non-o require medical certificate, insurance or criminal background check. Unless there is a compelling reason he would be far better off applying for a non-o from home country. Less hassle and less documentation required.
showing your 800k seasoned for 2 months in Thai bank, long term lease agreement. Go to immigration and get their document checklist. They can vary slightly. No need for an agent.
What is your non-o based on? Marriage may have hone visits and lots of documentation requirements, but retirement has mostly residence document requirements.