The loss of a baby is hard to bear. I am sorry to hear of his loss.
If you are offering your home as a safe place for him to be at for the initial few weeks while he is grieving, that is one thing. Offering your body, your lovers arms to comfort -- that is another thing. Offering you and your husband in a live in "V" -- that's another thing too and not appropriate at this time.
You can WAIT to offer that increased level of commitment in your tier of relationship when he returns to stable ground in himself. You don't fuck with fragile -- that's not kind to do.
Changing your relationship level to him when he is NOT mourning maybe two things is better -- mourning the loss of his potential baby, and mourning the loss of relationship with the mother? (You do not specify where that is at? I assume they are not together any more otherwise he'd be grieving the baby loss with her?)
It's better to be a friend
right now -- that's what he sounds like he needs most. You with your "friend hat" on.
It is normal to crave sex after a death -- the human need to feel "yes! Loss happened but *I* am alive!" and since you are "sometimes lovers" maybe you both choose to indulge in that and create a space to be that kind of comfort to him too -- the warm body comfort of a lovers arms. Schedule a vacation
, not a larger (possibly permanent) life change in times of duress.
Times of duress are not good times for making Big Life Changing Choices. If the desire to update your relationship to a live-in V thing is there now, it will still be there a few months down the road when he is not mourning. A change like that is involving your husband too. If you are not taking advantage of him now, you still won't be later either. So wait til later til you are sure you can take his word at a time when he is NOT suffering other losses. A better time than the mourning time.
Just as it is not decent to take up with a man who just lost his wife and is a widower, it's not decent to take up with a man who has just lost a partner/baby! He might SAY he is ok, but his circumstances right now could make his judgement clouded.
You guys being in a V could be the right thing, but you can have the right thing at the RIGHT TIME, right? Wait til his judgement is not maybe clouded from mourning a death.
And be also looking out for YOUR best health -- not just his.
"We became a V because we saw we really wanted to be together: Both in times of bad emotional weather, and in times of good emotional weather."
is a much better reason and foundational start to a live-in V than
"We became a live in V because he was upset over his break up/loss of his baby and thought he wanted this at the time and I was so eager to be that. But now that the storm has passed he's flaking out on me. Ugh! "
Your husband reaction is not "Yay!" but "I think he's just wants familiar -- but alright then" -- that's worth noting.
If a live-in "V" is what you want, give that live-in V the best foundational time you can give it to start it up in. Not be giving it a crap time to start it in.
So be a friend for sure. Be his lover, maybe. But be his live in V partner? That part of it can WAIT.
Just my 2 cents,