Hi, I'd like to help you with your question.
It is very common for children to respond to trauma of any kind by reverting back to bed wetting. Children respond to trauma in different ways. They are unable to express themselves as well as adults can, and they are limited in what they can understand. At the age of 6, children are very literal in their ability to understand. They also do not grasp that someone is gone for good. So they keep wondering when they will be back.
The best option for treating this problem is therapy. Although therapy cannot directly address the bed wetting, it can help your son through the grief over his mother's death. Once he can cope better with the loss, the bed wetting should stop. In the meanwhile, continue to work with him on staying dry at night. Reinforce what your wife did to train him originally and add some of your own techniques. Try to be gentle and non judgmental. It is frustrating for you and that is understandable, but he will be able to keep himself dry at night once he copes with the loss of his mother.
To find a therapist, ask your son's doctor for a referral. Or you can search on line at http://www.apa.org/ or http://www.findachildtherapist.com/. Here are some guides to finding a therapist:
Here are some other resources to help:
Help Me Say Goodbye: Activities for Helping Kids Cope When a Special Person Dies by Janis L. Silverman
Children and Grief: Helping Your Child Understand Death by Joey O'Connor
When Children Grieve: For Adults to Help Children Deal with Death, Divorce, Pet Loss, Moving, and Other Losses by John W. James, Russell Friedman and Dr. Leslie Matthews
You can find these books on Amazon.com or your local library may have them for you.
I am so sorry about your loss.