It sounds to me like this all needs to be out in the open with as much honesty as possible.
If I were in your situation I would sit them both down, without the influence of alcohol or anything else and say that you have been reading up on polyamory and found it to mean that people have more than one relationship with others. Its not just about sex, but about love and caring for one another like partners do. That you see the three of you working towards that if everyone is game but will need to proceed slowly and with consideration for everyone involved to make sure everyone feels safe and part of the process. You would all need to be honest and forthcoming with how you are all doing too.
Then try a date night with each other seperately and together. Work on scheduling so that your time does not become unbalanced and go from there. I would hold off on sex until you get a routine and comfort level that is not based on fucking but on being partners to one another. Then add sex. Sexual energy can build. There is nothing wrong with that I don't think, its about appropriate timing though. Have a look at some threads here in the search engine, "unicorns" "secondaries" "secondary" "foundations" "lessons" they are all good ones to gather information on how others have failed or succeeded and what they have learned. Do your home work and get them involved with that.
Of course he might just want to fuck two women together and that's it, or have both of you as his friends with benefits. This is where honesty comes in. You will all have to decide where this is going and be sure to set your boundaries around that. If you are not okay with something, say it, and don't feel you have to do something that doesn't feel comfortable.
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