Any suggestions on how to reduce howling?

I adopted Geoffrey in April, from a shelter that had rescued him from a puppy mill. He is doing great- has stopped chewing everything in sight, is housebroken, etc. Our only remaining problem is that sometimes when I leave for work, Geoff will howl for hours. Many times, but not always, he is much better when I leave him with a Kong or a bone, which is what I have been doing. This is a concern because i live in an apartment building. My neighbors all love Geoff, and I want them to continue to love him!!! He is two years old. I also have a cat, but I'm afraid she is not much company for him, as she is elderly.