Sorry to hear about your dog – I had a dog with similar issues (turned out to be a brain tumor). When I left he would howl and pace from the front door to the steps to the back door to the front door. It was really sad. Valium helped a little but the only real solution was to have my retired parents take care of him.
Someone who works from home or is retired might be a good option. Sounds like he doesn’t need much more than company during the day.