Boy, have I ever been there done that! This happened to my own son two years ago. I had the same thing happen to me (twice!), and I think it was actually more painful to see my son go through it than going through it myself!
It's good that he called you and shared his feelings. My son chose to deal with it himself and kept what he was going through inside and it was NOT good. I agree with the others to try not to badmouth daughter in law, as hard as it will be. No matter what happens between the two of them, she will always be the mother of your grandchild and you will have her in your life to some extent, whether you like it or not! And they may work things out between them and then YOU will be the bad guy! The best thing you can do now is be available to LISTEN and let him, vent, vent, vent, as much as he needs to while he sorts through his feelings. Having a child between them makes it even more difficult to deal with.
Give him as much encouragement as you can and let him know you are there for him. My son's first thoughts were that it was something that HE had done that caused it, blaming himself. It took quite a while and a lot of people talking to him before he finally realized that it was HER ... he had done nothing wrong. He finally got it that it was not caused by something he had done or not done or because of some shortcoming he had ... it was HER and it was NOT his fault! Finally realizing that was what seemed to be the turning point for him and put him on top of the situation. He went from being hurt to being :mad:MAD :mad: and then I knew that he had finally turned that corner and was going to be alright!
Sending lots of hugs to you and your son.