L.W.
Sounds like separation anxiety to me-thankfully none of my children went through that. But I have one little daycare child who is going through it now, though. And one thing I find is the hardest is for the parents to just leave their child and walk away. SOME parents have the tendency to feed into the child and their crying fits by lingering. I find that it's best to reassure your child that they will be okay, that you will be back, give them a big hug/kiss and go on about your way. Start developing a routine and the child will begin to see that "mommy/daddy is coming right back to get me and I'll be okay" and the crying and screaming won't even be an issue.
You have to get him comfortable around other ppl. Start taking him out more so that he gets used to different ppl. Start going to some mommy and me classes, or take him to the Gymboree, Stay and Play, etc so that he can get used to being around other ppl. And if he's crying the entire time with the babysitter imagine how tough that can be for her-to watch a baby that cries all the time. It's quite nervewrecking especially when it's a situation that can easily be turned around. Good luck!~