Hi E.,
I am going to mimick the thoughts of other moms and suggest counseling. My sister is actually a practicing Social Worker in Michigan. Her specialty is Child Development. You CAN find therapists who specialize in children. As already mentioned, with smaller kids the therapy is often through play. You should see her office... it's full of TOYS! :-) But through the play the therapist knows what to look for, what types of questions to ask, and knows how and when to probe for more. It's very effective. In addition, these specially trained therapists will often have certain sessions where mom or mom and dad are there with the child; or they will meet with just the parent(s) to gain more information and give insight.
I think in a case of divorce it is beneficial. Often times during divorce the parents just don't know how or what to tell the kids, or how to handle it when the kids aren't dealing with the situation well. And, in cases where safety is concerned it can be even more difficult. Kids often think things are their fault. They did something wrong and now their world is upside down. And really, it's understandable. We spend so much of our time telling them what not to do, and all that....
A therapist could offer you insight, tools, and teach you skills to help your son. In addition, sometimes... it's easier for kids to talk to a third party. They aren't afraid of 'making them mad'.... like mom or dad.
My heart breaks for you. I know divorce is never easy... and going through it with a child... must be very tough.