I dunno, what if he is in love with her? Really he wouldn't know until he is through his grief. I would be request working on that first rather than threatening to leave.
I think if I were in your situation I would request (no one can 'make' him) that he not see this woman for, say, a year. I would ask that the two of you work together to get through your grief without any out side "help" other than therapists and counselors. I would ask that he do his own work on his relationship orientation. After the year, I would ask him to revisit his feelings and let you know where he is at. If he is still grieving as much then he needs more time, if he still "loves" her then its time to look at where your relationship is going in the future.
You had a child together, you lost that child together, its time to grieve together. I'd be pulling his chin around, looking him in the eye, telling him I love him and that we need to do this together and then holding him and reminding him of what you lost together. He has a responsibility to that. He's lost his way here and needs a reality check. That's all that matters here, she doesn't. It seems to me that dealing with your grief is where you should both be looking. Not at poly! She can wait. He can politely tell her that by saying he has to do some work with her now. "Catch you when that's done," kind of thing.
Sorry for your loss by the way. How horrible to lose a child. I can't image how horrible that would be. Be strong for all of you right now. It sounds like he needs you to be so he can pull it together. The mans lost I think.
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