OP, addressing the only child thing: I'm an only child. I would describe my childhood as idyllic :) My parents facilitated a great social life for me, and i never for one moment felt like i was missing out on anything. One example (to get down to nitty-gritty details, which might help) from quite a young age, i was aware of how friends with siblings always seemed to have half of their stuff lost, broken or being taken off them by their brothers and sisters and it was a bit of a pain trying to play with it properly. I had friends over to play all the time - i had lots of toys and it was lovely to share them - but you know what? I was always glad at the end of the day to have my stuff back to myself and have the whole set of x,y, z to play with whenever i liked.
I've known a lot of adults who have dreadful relationships with their siblings. One of my oldest friends feels her young life was practically ruined by her older brother and is only now, in her 40s, getting to grips with her past. My XMIL used to bitterly row with her sister and once they got to their 40s they went completely non contact.
OP, i know it's swings and roundabouts, and that there's much more to this than simply wanting to give a sibling to your DS, but it does sound as if this is a big part of it for you. Please take heart from the truth of the matter which is that every family is different, there's no such thing as the 'One Blissful Perfect Upbringing' and the happiness of your DS does not hinge upon having a sibling at all.
Worrying about the worst happening and being left with no children is one of those sort of intrusive nightmare thoughts which we all have and it's shit.
My perspective of all this comes from being an only myself as i've said, and with both my parents being only children too (and perfectly ok with it). I went on to have 3 DCs very close in age when i was in my 20s. There are definitely good and bad points about that for the 3 of them when i compare their childhoods to mine. Now i've had a fourth with my second husband (took four years to conceive - so take heart there too on the ttc front) and fourth DD is being raised very much like an only, as the gap is so big between her and her sisters. I'm not worried about her, because i've been there and i loved my childhood, everyone has their own story in the end.
Sorry for the garbled post. Stream of thought in a rush 