You don't say what passport your son holds but if he is coming on a tourist visa or visa exempt you may have to show a return ticket. Some airlines require this before they will allow you to board & it is rarely asked for at immigration. Try googleing onward ticket & book anything out of Thailand. Don't forget that immigration may ask him for proof of accomadation, so an invitation from you & you can also be asked to show proof of funds, I think
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BT or equivalent.
With regard to if he wants to stay longer as a dependent, I think he would need to have been legally adopted by his step father to be able to piggy back on his visa as you have to show proof of relationship. At 15 I assume if he stays he will go to school so may be able to apply for an education visa. You don't say what visa you hold so would it be possible to be your dependent?
I think it would make sense to talk with your local immigration office to see what they suggest or call the Thai embassy in the country he is from.
Thai friend did this trip a couple of months ago, flight booked to Dublin but UK visa. She was denied boarding & had to rebook via heathrow which was both costly & time consuming.
I'm actually in Hua Hin but yes you'll find expat communities in most areas. There are alot of gated communities here that are mainly expat/thai couples the best area depends on what you are looking for as we are all different, consider what your average day will be & what you'll spend time doing.
While I like Chiang Mai I would struggle with the air quality during burning season, for me Hua Hin has everything I need.
800k each unless you are married in which case 1 retirement & 1 dependant/trailing spouse. You will need to get your marriage certificate legalised & translated for use in Thailand.
If you're worried about exchange rates you can have a foriegn curreny account with most Thai banks & acceptable by Immigration. Your problem now if you did this would be showing the money came from overseas
If you qualify for the LTR then great otherwise you have 2 real opions, either an O or an OA visa.
The OA is multi entry & is applied for in the uk, needs health insurance & proof of funds in uk account along with police check & doctors cert. All the information is on the Thai embassy website & it's not as complicated as it 1st seems. To extend you will need Thai bank deposit or 12 monthly deposits & health insurance.
The O visa can be obtained in the Uk or in thailand valid for 90 days. You would then go to your nearest Immigration office to get a years extension based on retirement at which point you need to show 800k in a Thai account which has been seasoned for
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months. You don't have to have health Insurance but it's still a good idea. Both of these can be extended 12 months at a time & it is possible to have a multi reentry permit or to buy a single re entry permit.
You can use the monthly income route but only on your second year extension as you must show a deposit of 65k every month for the previous year.
You are not allowed to work here on a retirement visa but if you're really just answering emails then it shouldn't be an issue.