I think a social group is an awesome idea!
Our DS is 11 months old and sooo many people have assumed we were having another - they started asking us when we would have another when he was just 6 months old! We are quite happy with our lil dude and he rounds out our family quite nicely. I love being able to give him my time and attention, while still being able to spend time doing things around the house, with DH, our friends, and engaging in hobbies, travel, etc. We can take DS anywhere we go - music festivals, art galleries - strapped to me or his papa. We are able to have more couple time (with DS along for the ride) as well as expose DS to so many more things than we could if we had two or more, just by bringing him with us. Also, as an extra bonus, it has also given me the opportunity to re-evaluate where my career is headed, and what I want to use this time at home for to set me up to be happier down the road (if I go back to work/school - or, alternatively, stay home and homeschool).
I do think sometimes siblings can be great friends, but they can also be great enemies, so it's a toss up - not necessarily a strong reason to have more, though in certain situations like yours, mayaandx, I can totally understand why you might wish he had siblings. I will keep you in my thoughts and prayers - you are in a tough spot. I can't speak personally, but I certainly think that your love and attention will make all the difference in how your DC navigates this time in his life. I haven't really explored the geographic tribes on here yet - is it possible to find moms on MDC who might live near you? Some of the homeschooler threads on here have addressed this - finding social opportunities for onlies. If you search there you may find something helpful. Wishing you the best!