By asking (it seems like demanding though) her to let you be poly, are you also saying "Do it my way." When you got married, and signed a legal contract, she was under the impression it would be a mono marriage, and now you have asked her to change the game after more than a decade of mono tradition.
If you want to make this work, I agree that you have to make your wife feel loved and appreciated, so she doesn't think it is because she is inadequate for you. As for your son, I can understand how it may be difficult. Sadly, choosing to be poly after so long with mono woman would be considered an irreconcilable difference with logical grounds for divorce on her end.
If you can convince her she is still loved and respected, she may begin to be more open with you, and if you do divorce, it may make the process easier. If she loves you, she should be willing to either let you be poly or divorce you kindly without cutting you off from your son. Seeing a counselor is a great idea, and going together would be even better if it would help the two of you communicate better (mostly so you can get through to her).
This may not be what you wanted to here, but it sounds like you are blackmailing her as much as she is you, and fighting anger with anger only creates pain and resentment. Try love, and lots of it. Ask her what she needs, think about her concerns, and try to find some common ground.
Best of luck,