What is considered a public charge that could risk the status of a green card holder?

The public charge provisions is for people applying to become permanent residents. Here, your son is already a permanent resident.

The new rule seems to focus on cash assistance to allow someone to simply survive in the U.S. (like food and housing).

A financial award at the college level for permanent resident students is generally not considered a public charge. Also, it will not jeopardize your status in the U.S.

No. This will not be categorized as a public charge item. My understanding is that even if it were to be categorized as a public charge item, that would not have affected you since you already have your green cards as a family. This is a consideration for people who are currently applying for a green card as opposed to the ones who already have green cards. Going to a public university is not one of the "public charge" categories, anyway.

For purposes of determining inadmissibility, "public charge" means an individual who is likely to become primarily dependent on the government for subsistence, as demonstrated by either the receipt of public cash assistance for income maintenance or institutionalization for long-term care at government expense. So receiving financial aid for education should not fall under "public charge." And since you are already a green card holder, the public chargeability issue would not arise.

This will not lead to a determination that you have become a public charge.

I have not seen that on the list, but the new regulations are about 900 pages long.

Not at all.

This is an evolving area of the law and, to a large degree, it remains to be seen how PC will play out. Education does not seem to be the area of concern, but you should connect with your attorney and get a legal opinion addressing your specific situation.

Probably not.