It's very possible that he requires reassurance due to the loss of his feline companion. I know for a fact that animals do go through a grieving period though it is not known whether it actually grieving or just a period of readjustment. I think they know and I interpret this behaviour as grief.
I guess try to maintain your normal routine and things should fall into place in time. My mother's cat whom died last year, left a void in the life of the other cat. Though she is no longer 3rd wheel in the animal household, she has declined in activity and now health. Something is to be said about the loss of our pets. I truly think that both humans and animals equally feel a void.