Poor Barnaby! I guess he wants to eat that stick, huh? He doesn't want cuddle time?
We have 4 cats and have added them all one at a time. Two are littermates, but we brought one in first, and then the other a month or so later. Everyone had adjustment times, but I honestly think most of them are happier having each other around to play with. They aren't overly affectionate with each other, but I think it helps with loneliness and boredom.
If you decide to try another cat/kitten, give it plenty of time. And be sure to give Barnaby some alone time, since a kitten is overwhelming in energy. Our poor male adult became very fatigued with our new kitten b/c he wouldn't make the kitten leave him alone!
If Barnaby seemed unhappy after a month or two, you could consider finding the new kitten a new home.