My son lives in USA. He divorced his wife in the presence of two Egyptian trustworthy Muslim men. My son did not take back his wife during the ‘idda (post-divorce waiting period before which she is allowed to re-marry) and he afterwards married another woman. What is the ruling for this divorce in Islamic law?

This is a first revocable divorce. The marriage ends with the expiration of the ‘idda if your son did not take back his wife. This divorce is sound and compatible with Islamic law.
This is in answer to the above and God the Almighty knows best.