Awww this is sad to read, bless you. I can imagine just how you feel but you know, I think that as parents we worry about these things waaay more than our chidren themselves do.
Please, don't ever lose sight of the fact that for only children being only children is totally normal to them. They don't know any different.
I think there is so much pressure on parents of onlies who see (what apprears to be) everyone else with more than one child and automatically assume that these children are constantly amused and happy playing with their siblings. The reality is often quite different however. I have a neice and nephew two years apart and know many more sets of siblings who fight like cats and dogs, don't get on, annoy one another almost constantly, bicker, squabble, fight, have very little in common, are desperate to find other kids to play with and drive their parents spare with it all!
Besides, your DS is probably far more happy with his own company than you might imagine he is. Let him find his own level. Perhaps only try to arrange for other kids to come over if your DS specifically asks and don't force the issue particularly if it's causing you no end of hassle...? Have you tried talking to him about it? I have often said to my DS you can invite anyone back that you want, or do you want to ask "X" over and he's said "nah" but when he wants someone back, he'll ask soon enough and then I'll arrange it. Maybe leave it up to him more.
What about getting your DS into outside activities? My DS is quite busy during with the week and at weekends with a lot of sporting activities (football, swimming club, tennis, gym) where he sees other kids, he sees his close to him in age cousins too quite often and he mixes with other kids all day at school so he's often quite happy to be at home doing his own thing after all that.
He speaks to friends on MSN/Facebook and generally on the computer in other ways via games etc and he texts them and so on too so I feel that all in all he has plenty of interaction with kids. Kids are no different than adults in that they too need to have their own time and space and don't necessarily need every moment filled with other kids just as we don't want people around us all the time or want our all our spare time organised for us. Sometimes it's just nice to be on your own and do your own thing.
Don't forget too that the thing that most children with siblings crave is quality time alone with their parents and that's what your DS has in spades. The grass is always greener but it's all about finding a balance.
My DS seems perfectly happy with his life and I think we've got the balance right for him as an indidivdual. At almost 10 it won't be too long now before he's off to secondary school and will be old enough to make his own arrangement to meet up with the friends he chooses anyway. Even now, he arranges to meet his friends to go swimming and I drop him off and pick him up from the leisure centre so he has a bit of independence.
Don't worry and don't stress. I bet your DS isn't half as worried about this as you are.
Please come back and have a rant here if you want to. It can be hard but in reality you are probably putting far more pressure on yourself than you need too.
I also wish I lived closer to you too - I'd love to have you and your DS over for a "playdate" (YUK - I hate that word too, it's so flippin "yummymummy)!