A.G.
Just like you thought - this is a phase. I think all baby's go through this sometime between 9 and 18 months. You just need to keep reassuring and walk out. I bet he stops crying about 5 minutes after you are gone. One thing that does help is being prepared to leave immediately once the babysitter gets the information you need to convey. The longer the leaving process takes, the more the baby has time to get worked up and the longer it takes him to calm down.