V.T.
Oh, how familiar. We went through this too - and survived it. Chances are, something is worrying him - teething, illness, nightmares, digestion, perhaps a new skill he is trying to master. How long has it been happening?
Keep reassuring him so he won't learn to dislike going to sleep any more than he does now. Like Beth, I found Elizabeth Pantley's book interesting. Please don't jump into crying it out - it's probably not what he needs. Although you are thoroughly exhausted right now (I sympathize), keep helping him and soon his patterns will change.
You could also try increasing his activity during the day - I find my son sleeps much better after an active day - especially after time spent outdoors.
Good luck!