the only difficult part is opening a bank account. He already has a bank account. It's very easy, I did it myself inside Thailand, it's nearly the same as an extension. What is so difficult?
For the first year you need to follow the rules around 800,000 / 400,000 as others have stated. When you apply for your second extension, if you have deposited 12 x 65,000 monthly amounts, your second extension is based on the 65,000 method. I believe you can then withdraw the 800,000 immediately. Someone will correct me if there's an overlap period in the second year.
yes, I forgot to say the immigration people are very helpful. If you dress appropriately, behave politely and have all the documents ready and organised it's a very easy and stress free task. I have extended my stay 3 times now and on average it takes me about 45 minutes.
Yes it is a requirement, but if someone is coming to Thailand that doesn't have a sum of money for an emergency, it makes sense to have it. I don't know how much it costs as it will depend on the individuals background (age, existing medical conditions etc).
I think immigration are only doing their job, but I hope there is a way you can continue to see your son.
As Anna suggested, can you get the 'visiting family' visa or failing that, what about getting a non-OA visa as that doesn't require a deposit in Thailand?
Get organised with a folder with photocopies of your documents and some passport size photos.
Get the list from immigration and the application form.
You will need to get some documents from your bank, but they know what to provide.
The form to apply is one a4 size with about 15 simple questions (name, passport number, address date arrived in Thailand etc). You will probably need to draw a very basic map of where you live too.
Go to immigration when it is quiet, so avoid Monday and the day after bank holidays. Pay the 1,900 baht fee, wait a few minutes and it's done. Good luck.