Separation Anxiety in 3 month old male kitten

My husband and I recently adopted a new kitten from a rescue group in our area. He was actually born in his foster home but he is having some major separation anxiety issues. I'm not sure if he was separated from his mother too early or what. Anyway,he follows us all over the house and cries when he can't see one of us. When we go to leave he cries for at least 30 minutes and I'm sure it's stressful for him. This morning he sat outside my shower and cried because although he could see me, he couldn't get to me. I've tried putting him down in his bed with one of my sweatshirts, waiting until he falls asleep and keeping him in the same room as our pet rabbit. But nothing seems to make him feel any better except when we come back in and let him out and then it's all purrs. I'm really starting to get concerned about the little guy. Does anyone have any other ideas how I can make him feel more secure?

I really like Sally Bean's answer and hope you might get another kitten companion for the little guy. And you shouldn't get overly anxious about HIS anxiety - it's something those little tykes have to go through.

You could treat him with the Bach essences Walnut (adjustment), Chicory (demanding constant attention), Mimulus (fears of separation or strange circumstances). They can be purchased at most health foods stores and if you get a knowledgeable person there they can tell you how to make up a "treatment bottle" with all three of these in it. Some store personnel are more knowledgeable than others. I ran into a gal the other day in a new store who not only knew a great deal about them and other "herbs" as well and she knew they could be used successfully with animals as well as people.