I previously have booked tickets with Thai Airways which have a departure and return date. I have then cancelled my return flight, and booked another date, sometimes 2 to 3 times. There was a fee of 500 baht each time I did this. Not sure if it is still the same as I haven’t done it for a while. Maybe contact Thai Airways for confirmation. I’m sure it would be cheaper than a flex ticket. Good luck.
You could book a return ticket with Thai Airlines, putting an approximate return date, then change the flight when you know your return date. Not too expensive.
My understanding is that he can get a passport. He has to do it in Thailand, and be included on your wife’s family book. Unfortunately, being the holder of a Thai passport, he would be eligible for military conscription.
I think it depends on age. My 17 year old son has been told he has to be registered in Thailand before he can get a Thai passport. He was born in Australia, they gave him a Thai passport in Australia when he was young.
I used a visa service center in Pattaya for all the visas for my now wife. Very efficient, not too expensive, and took away a lot of anxiety for her and I. I think you can find them in most major cities.
I believe that children born overseas need to included on the family book in Thailand before they can apply for a Thai passport. This was not the case previously, and can only be done in Thailand.