I live with my son who provides some cash support to both me and my wife. There is no W2. It's simple cash and not in exchange of any work. How do I report this on my 1040 (married filing jointly)?

I do get subsidies from government for healthcare market place account.

Your son is making cash gifts to you, not paying you a salary. This does not need to be reported to the IRS, unless the amount he gives you is greater than $14,000 x 2 = $28K, in which case your son (not you) would have to file a gift tax form.

What ebaines says is exactly right.

Thank you so much. So, that means I have zero income. I actually get $50 a month from my employer back home from where I retired. While I can report that, its just too much paper work.

Since I am getting subsidized healthcare, I must file my taxes. Is writing $0 in income okay - when I applied for healthcare subsidies I included my pension plus my son's cash support in my total income - will they see a discrepancy - my total annual income remains less than $10000, hence the reason for living with my son?

Report the pension, as that is in fact income, but NOT the cash gifts from your son.

Thanks. If I report the pension, do I also attach my pay stubs (it's really a book where each month has an endorsement) and then show the conversion of currency? What's the acceptable way of reporting currency conversion?

No, just report the figure. If the IRS wants proof, they will ask for it.