You should seek the advice of an estate planning attorney to help you with actually doing this properly. But I will give you some general information.
US green card holders are largely treated the same as US citizens for US tax purposes.
Gifts received. are not subject to US income tax. I.R.C. 102(a). https://www.law.cornell.edu/uscode/text/26/102
The US does not have an federal inheritance tax (on receiving not making a gift) so that is not an issue. A few US states have a state-level inheritance tax. So if the son lives in Iowa, Kentucky, Maryland, Nebraska, New Jersey, or Pennsylvania that might apply. https://www.thebalancemoney.com/state-inheritance-tax-chart-3505460
Gifts received from outside the US worth more than $100k must be reported on IRS Form 3520. This is not a tax, just reporting. https://www.irs.gov/forms-pubs/about-form-3520
The parents will be using up some of their $12 million each estate/gift tax exclusion. They will need to file an IRS Form 709 will need to be filed to keep track of that.
It is also possible that there will be some ongoing (yearly) information reporting of the foreign asset. I am not sure about that for real estate. If the asset is held in an entity (corporation, trust, etc.) that would be another reporting possibilty.
Also note that if the property is sold by the son, his "basis" or cost will be whatever the parent's basis or cost was at the time of the gift. This is called carryover basis. When sold his gain would be sales price - cost of sale - parents cost basis - any capital improvements made by the son.
If instead the son were to inherit the real estate when the parents dies, he would get a step-up in basis to the fair-market value as of the date of inheritance (death) and therefore not have to pay income tax (capital gains tax) if he were to sell. (because step-up basis is usually much higher than carryover basis). But of course there can be plenty of non-income tax reasons for doing such a transfer while the parents are still living.
I hope that helps a little. Be certain to obtain the advise of a profession such as an estate planning attorney who has worked in this area before to help you will the details and application to your specific facts.
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