This is tough and sadly common. Many guys, including me, get married to a woman they love in the hope they will be 'cured'. We can't be of course. There is nothing to cure.
You will find that they cannot give up their need to express herself, with an inner turmoil that is beyond belief. I would keep an eye on them, they may start drinking, and end up drinking heavily. They may start to show signs of depression, and the associated harms that go with it.
They love you and their son. It is ripping them apart and always will.
Maybe 'cross dressing' at home may help. But to be honest I considered cross dressing when I had to wear male clothes to go to work, female clothing was just normal.
They may lose sexual interest in you, this happened with me. I loved and still love my wife but I'm heterosexual, I couldn't keep trying to have sex with another woman, no matter how I looked physically.
Is there a solution? To be blunt, probably not. There are a number of couples here who married had a family and one of the parents had to face the fact that they were a trans*person. For some couples it works, for many it doesn't. Some love and care for their children in a family life, but cannot have a sexual or physical relationship ith each other. BTW children tend to be very accepting.
You need frank and honest discussion, I would suggest you both seek an experienced gender therapist to help with and to guide the discussion. All three of you in the family need to be honest.
He may be trying very hard to hide how he thinks and feels, but she will dominate in the end. She has to, she is him and will need to live.
Hugs Honey and Please ask anything.
Do join the Significant Other area as well as there are couples there who may be able to help.
Cindy