When your son shares his fears and feelings with you, you need to validate them.
When we sat our D's down and talked to them, we told them that in the course of a marriage because of frustrations and unhappiness sometimes parents separate and divorce. It isn't any one person's fault and that both their Dad and I were to blame. That they had no fault in it at all and they were the best of our accomplishments and how very much we loved them.
My D's know that I love thier Dad. It doesn't mean we can be husband and wife though.
I used the movie "Lilo and Stitch" to help talk about the difference in our family with D's.
I know there are others here that can help you with their experiences too.