The reluctant co-sleeper???

I'm a major lurker here but have finally come to my wits end about sleep issues with out 18 month old ds. A little background: G slept with us for the first few months until we gradually started the evening off with him in a crib. He'd never go very long (maybe 2 hours, 3 if we were lucky) until he woke and we brought him into bed. This worked well for our family until he was a year or so.

Since then, he does not sleep well with us. In fact he wakes very frequently (think every hour or so) crying and fussing. He insists on being on my arm (which wouldn't be so bad if he actually stayed asleep) and is just generally restless. I know he sleeps better by himself. But he doesn't want too. He wants to sleep with us but keeps us all awake. Last night he went to bed at 8 or so, woke at 10:30, I held him for a while to get him back to sleep, put him to bed, he fussed a bit but went to sleep, woke again 20 minutes later and was up until 3:30. He would not settle down and was obviously tired. He woke this morning at 8!!!

We have had bits of time where he does sleep for long stretches of time or even, gasp, through the night. But they seem to be few and far between. Any ideas or advice? Thanks so much for helping out this sleep deprived mama.