I'm approaching 30, I'm not in the sort of relationship where I'd be ready to raise a child with a partner nor am I financially prepared to do so, my mother had great difficulty having a kid (her first child was stillborn), and I have polycystic ovarian syndrome. Because of all those factors, while I have no cause to think it's impossible for me to have a biological child, I also accept that it might never happen.
For that reason, it's been a wonderful gift to get to be so involved with Gia and Eric as Bee came into their lives. Being there through the pregnancy, meeting the baby on his very first day on earth, getting to know and love him as he's learned to laugh and crawl and try new foods and cut his first teeth... and, I hope, all of the wonderful milestones ahead... all of that is a unique and powerful part of the human experience and thanks to my closeness with G&E I feel like even if I never have a kid of my own, which I know would take it to a new level, at least I haven't entirely missed out on it.
With Lin, as you said, if it were just the two of you, you wouldn't be thinking of having a kid, and with his health the future is no guarantee. Getting to be there with you and Sward and the new baby may be a shot at experiencing some of the magic of this whole process that he would otherwise have no opportunity for. It doesn't sound like he's super excited about it at this point which is perfectly reasonable -- I spent much of Gia's pregnancy freaking out, over her health, the future, our relationship... there's soooo much to be scared of -- but I just wanted to put this thought out there to both of you, that rather than a burden this scenario could be seen as an amazing chance at something otherwise inaccessible.
And I bet you'd let him be there at the birth, too (siiiiiiigh... still a little sad to have missed that).
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.