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.
As Tony said why would you plan to come to a foreign country and start doing illegal things, would you do that in your own country? People will say lots of foreigners do this in Thailand, but that doesn't make it legal and people do get deported for doing this. I would be especially nervous doing it now at a time when the banks are being scrutinised. The rules are there for a good reason to ensure people have enough money in an emergency situation. If you don't want to deposit 800,000 why not get the OA visa instead and buy the mandatory insurance.