We really can't add much to what has already been said. For some, they may never get over that hurdle, for others - like us - it's a long slow trip into the pool
Definitely stop going behind his back. Whether or not your are only playing with women, that is still an affair in our book. If there is one thing that being in the lifestyle requires, it is unconditional trust. Without that, you can never expect him to share you with others.
In terms of "hands on" advice we offer the following: Keep up the conversation. Use fantasies in the bedroom. Direct him to these boards to read.
If you have them in your area, go to an "off premise" party (no sex allowed on premises). Go with the absolute understanding that there is no playing of any kind - even light stuff like kissing or dancing with other couples (or women!) on the dance floor. That will let him experience the "vibe" without any pressure. It will still be nerve wracking, but it's a nice soft introduction. Make sure you don't change your rules mid-stream, because you would rather give up an opportunity rather than live with next morning regrets which could really end your chances.
Best of luck!