He has a few options, the first 2 are probably easier

1. Apply for a 90 day Non-O based on marriage in the UK before he returns

2. Apply for a 90 day Non-O based on being over 50 before he returns

3. Return visa exempt and apply for the appropriate 90 day Non-O at his local immigration office

Once the 90 day Non-O is running down apply for a 12 month extension of stay

You can't have an adult child dependent on your DTV. It is possible to get an adult child dependent visa but you need to be on a long term non-immigrant visa first. It's also going to need medical certification and agreement from immigration so its a difficult option to get.

@Michael *******

I'd strongly advise you to go and get independent advice. It's not about the training it's what you do after you are trained. It might be worth you looking through some of the jobs here and seeing the requirements

No, only a 12 month extension is possible unless you have Thai family in which case you can apply for a 60 day extension based on them

@Michael *******

times change. You need to have a bachelors degree now to get the Non-B visa that allows you to work inside Thailand as a teacher. Here's a link to the requirements for immigration to issue a visa in Thailand, look at 5.5 and 6.6 /2.FOR-WORKING-AS-TEACHER-NON-B.pdf

So you arrived on a tourist visa, applied for and obtained a 90 day Non-O visa inside Thailand (which will be stamped used). Your visa is now not relevant, all that is relevant is the date you are permitted to stay in Thailand until. You can apply for a 12 month extension of stay (not a visa) when you have 30 days or less left on your permitted to stay stamp, some immigration offices allow you to apply when you have 45 days or less. Best to check with your local immigration office and also get their handout of requirements

No way to check

@Michael *******

do you have a bachelors degree? If you don't its going to be difficult to get a teaching position, TEFL is not enough. What the right visa is depends on your situation. If you want to come and teach you will first need to find a job and get your employer to provide the paperwork for you to apply for a Non-B visa

@Hardev ******

should be

Assuming you're applying for a 90 day Non-O based on marriage yes