I feel for you & your little pup. It sounds like you are doing the right things for both of you. I'm sure he is grieving & hope that time will help him feel not so sad, just like people.
This isn't the same situation, but I had lucy, by herself, for 4 years & then got pebbles. Pebbles was only 9 months old. It has been the best thing for lucy, she is more active, happy & actually even trimmed down some weight. Maybe a younger pup would not be a bad thing for your little guy.
to both of you....