How do I help my dog not be so lonely when left alone?

My beagle is 4yo. He's been living at my parents' house his whole life, and now that I'm out of college and have an apt that allows pets, he's coming to live with me. When I leave him at home alone, he gets extremely lonely. He used to have other dogs to keep him company when no one was home, but here he's the only dog. I think that he sits by the door and whines and howls the whole time I'm gone (even if it's several hours). I can hear him from the hallway outside, and when I open the door really quietly, he's already sitting there, staring at the door). When he sees me, he goes ballistic and starts howling, jumping, and wagging his tail furiously. He's also had a few accidents on the carpet, which is very unlike him and never happens when I'm home.