Possibly the biggest thing I can recommend is that you remember that he has the right to react whatever way that he does. If you for one second become selfish about it, I fear you will lose the point. This means, no whining about not getting your needs met, or anger about his over reaction etc. This is not a time to be the center of attention in this.
I know that will be hard as I'm sure you have thought about this and want to get moving. It could take a very long time and you will need to be ultra patient and prepared for that actuality.
I suggest that, like anything else in a good relationship, you ask him to explore what it all means "together." Make it your "together" project. Read books "together," discuss "together," move forward "together" and slowly change "together." All the while expressing your love for him and commitment to him as well as your fears, doubts with complete honesty.
If you don't communicate well and in an honest way already then you might want to make your exploration about that first.... how to communicate effectively. Without telling him about your poly thoughts perhaps even.
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