Alternative if you didn't go down the tourist visa route. Is to include a border bounce on the back of visa exempt entry (extended once). It's a lot more hassel and works out more expensive than a tourist visa. With the tourist visa, you'd also have more options available further down the line...
Let's pull the worst English I have ever read through a proofreading:
My son has lived in the U.K. for 18 years. He was born in Thailand but only lived there for 3 months. He wants to join the British Army, and they have asked for a letter of liability stating he has no commitment to the Thai Army. However, when you get in touch with the Thai embassy, they say all Thai men need to take part in the lottery draw. Yet, the British Army says he should be able to get a letter from the Thai Embassy in London stating he has no liability to the Thai Army. Please advise. Thanks.
Here are the changes made:
1. Removed unnecessary punctuation (" . . . .").
2. Corrected "get into touch" to "get in touch."
3. Combined sentences to improve flow and clarity.
4. Added a comma for readability ("join the British Army, and they have...").
5. Replaced "army. army." with "Army."
6. Clarified the embassy's location ("Thai Embassy in London").
7. Improved overall grammar and punctuation for clarity and readability.
What 90 day visa are you talking about? I think you are confused. A tourist visa is valid for entry for 90 days, once you enter you have 60 days stay before you leave or extend once, for a further 30 days.
You can apply for the retirement non-o after arriving visa exemption, although you have to get your skates on to get a change visa to non--o submitted with 15+days remaining on your stamp. Note you can extend your arrival stamp once 30+ days here in Thailand, to give you a little more time.
Start with opening a bank account once you are here. Use an agent if needed.