The therapist told me his wife has BPD and that my son should get out of the marriage especially before they have children. What do I do?
This, of course, is not a one paragraph answer - my advice is to get active here, learn what you are dealing with, how not to making matters worse, and how to help you son. Joining here would be good idea for him too.
First and foremost, don't pass a third hand diagnosis off to your son - if he tells his wife she will feel threatened and she may start pulling her marriage (and you son) away from you.
Rather than trying to separate them (ending the marriage has to be a decision he reaches), my initial focus would be keeping communications line open with both of them.
Learn about the disorder. Learn about the developments in the relationship.