I'm sort of in the same boat with Charlie, except he's 2, and I've had for about 3 and half months. He always had his parents and sister. If I leave him alone he sounds like someone is killing him ! Good thing I can bring him to work
Draining their energy is big part of it for sure. I've been using the advice from this book, "I'll be Home Soon - How to Prevent and Treat Separation Anxiety" (by Patricia McConnell)
We're up to 10 minutes now after about a month. Not much, but every victory is a good victory !