Does 0bamacare require a parent to pay for his/her 20 year old son's health insurance?

No it just gives parents the option. In some cases at 20 you are still in school. Parents should way the benefits in having their kids on their plan vs them being on their own.

If a parent has no insurance how can he cover the son ?

The law does not require a parent to add an adult child to his/her policy. That is optional.

Anyone over age 18 can buy insurance from the state exchange.

Links to subsidy calculator, Obamacare website

I have been wondering how that would work. And what if a 21 year old that is on her parents plan has a baby, who will have to insure the baby?

it's only an option... if the parent has insurance they can keep the child on it but do not have to...

there are no obligations that I am aware of in any law that states you have to insure your children at any age...

21 is around the age kids start to graduate from college... really the 21 age rates are probably the same for those younger...

No. The law just requires everyone to have some sort of health insurance, it doesn't matter where you get it. Through exchanges, medicare, private insurance whatever.

You see, the reason why the law came about was to address the massive amounts of money spent by States to cover uncompensated care provided by hospitals.

No it "allows" you the opportunity

edit do you want to have a stay in the "camps"

Yes.

it will all work out in the end, drink up