All good suggestions!
I'll add that its important to figure out the things that make him want to pretend like the conversation never happened. What are the feelings the bubble up in him that make this all just easier to forget?
When you talk to him next, start by asking specific questions. Its one thing to ask 'how do you feel about polyamory?'. That can illicit any number of responses. But asking questions tailored specifically to his own concerns might help focus the conversation. "Does the idea of being non-monogamous make you feel like I'm trying to replace you?" "Does the idea of me dating other people make you think that I somehow love you less?"
Make sure to reassure him that no matter how he answers these questions, his responses are valid and important. Once you have some specific fears widdled down to their essentials, you can begin to address what the problems really are.
Tell him flat out that you're worried about him side-stepping the real issues here, and you want to be clear about all of it.
If he tells you "I'm afraid that if you date other people, you will leave me for them." That's a result of insecurity, and a real risk in any relationship (not just poly ones). You can help quell this one by saying that the whole point of Polyamory is that you don't have to chose. You love him, and for as long as you love each other, you want to always be free to express that. Think about WHY you might want to date other people, and when you explain it to him, make sure you illucidate the points that indicate how it doesn't involve replacing him.
You can say to him also that you have no desire to focus on anyone but him and your child right now, but loving freely is an important part of who you are, and that its best to talk about it now. Tell him you don't intend to seek out other people walking into your lives at the moment, you simply dont have the energy to handle it. This way it doesn't feel like an ultimatum to him, like the conversation can take its time.
In any case, I'm sure this is the kind of stuff communciation seminars can help walk you through. Its a daunting process at first for people who aren't used to breaking concerns down that deeply, and especially difficult for anyone who's ever blamed themselves for feeling a certain way. Like the other folks above, I highly recommend taking a class together on it.