L.:
I'm a special ed teacher, so I do have experience with these issues. My first bit of advise is: Don't be discouraged! This will get better!
When you say that he was "diagnosed", do you mean by your son's school? If so, please make sure that he is accepted into the Special Education program, or will have a "504" status. This will put him in touch with the academic resources that he needs. There are programs that are specifically designed to address dyslexia. They are very comprehensive and systematic in teaching a child with dyslexia to read well. (Another plus: His handwriting should be beautiful!)
Irlen's Syndrom is characterized by moving, or "dancing" text. Here is a website with lots of information: http://irlen.com/index.php?s=index
Now, in regard to his state of mind, give him time. Allow him to soak in the information that he is going have to work a little harder to get what everyone else seems to get so easily. The advise in a previous post about you and your ex working with him and supporting him together is great! He needs to know that you are going to love him, no matter what. Then, celebrate his successes, no matter how small! They may be small to you, but to him, it may have been like climbing a mountain!
Lastly, if his emotional state does not improve, then he may need to have some professional counciling. It sounds as if he may be depressed. This is a lot for a little boy to handle...divorce, failure in school, and then, finding out that he "has something". Also, please know that childhood depression does not always manifest itself like it does in adults. He may show his through rage instead.
Anyway, hang in there. Things will get better. Take advantage of all of your resources. Be your child's biggest advocate and champion at school and at home. Learn all you can so that you can help as much as possible.
Good luck.
A.