It really does sound like seperation anxiety, esp. if you just have to sit there until he falls asleep. And the crying it out that you're sitter did likely didn't help. I'm in the middle of the same thing, so you aren't alone. The advice I get a lot is to just hang in there, give you LO lots of extra love and attention, esp. at bedtimes, and things will get better. During the day, if he's screaming because he's afraid to be left alone, bring wherever you are going and let him play there - don't be a taxi, but let him know you are there and let him see you. Also, don't forget about playing peek-a-boo. Talking to other parents who have been through this, it really does help.
Theressa