I have just discovered that my son who is now 19 sexually abused my two daughters. Who are now 17 and 15. Abuse took place apparently from ages 6-12. Is there anything that can happen to him now legally?

Depends on a) where you are (laws vary) and b) whether your daughters press charges.

What do your daughters want to do? Was he 12 when he stopped? What was the nature of the abuse? At those ages it could have been simple playing doctor, or it could indicate a more serious problem.