I'm sorry to hear what is happening, it is heart breaking.
Instead of telling him why you left, try asking why he is in it. Try to get him to see that it is not the doctrine, or the brotherhood, but because he was born into it. Say that if you and his mother were Mormons, he would still be a true believer, but in what Mormons teach. And if you had left the Mormons, he would be looking down on your decision not to be a Mormon. Religion is a matter of birth as much as the language he speaks, and it should not be allowed to come between family.
No one ever made that point to me when I was growing up, yet I could never get out of my mind that I was only a JW by accident, and would never have converted if a couple of people arrived at my door offering magazines.