We live in LA, and my son is a green card holder. He just got admitted into college abroad for 3 Years. How long can he stay outside of U.S. Is it Ok for him to return to U.S. within a year and stay for a week vacation and go back to his school? Is it OK to do that repeatedly for 3 years until he graduates his college?

That will depend on the policy of where in abroad he will be studying.

We live in LA, and my son is a green card holder. He just got admitted into college abroad for 3 Years. How long can he stay outside of U.S.? Is it Ok for him to return to U.S. within a year and stay for a week vacation and go back to his school? Is it OK to do that repeatedly for 3 years until he graduates his college?

This is probably not a very good idea since being outside US for a long period may jeopardize his legal status.

He can file for re-entry permit which will allow him to stay outside US for up to 2 years but after that he will lose his legal status. If he wants to return to US after 2 years, he will need to obtain returning resident visa from US consulate in that country. To obtain this type of visa may take up to 6-8 months.

To file re-entry permit, he needs to submit form I-131 and gets approval before leaving.