Do you know another cat he can 'visit' with? My kitty Spook is an only cat, but I often take her over to spend a few days with my mum, and she plays with Zan, my mum's cat. The first visit they were stand-offish, but Spook has swiftly established herself as the dominant cat, despite being half Zan's size. She thinks she owns the house, and has demonstrated this by chasing Zan out the cat-door and refusing to let him back in. (Spook is an inside cat herself, and only allowed to go outside when I'm with her).
In my experience, neutered male cats tend to like being around female cats, and ours have always settled into friendships pretty fast. Usually, the females seem to be boss cat. The one exception would be one case where my mum got two kittens, Zan and his sister Kirri, and her pre-existing cat 'Little Cat' imediately moved in with the neighbour (who already had two cats anyway). Friendships tend to last a lifetime, and early behaviour patterns often perpetuate. Zan used to suck on Kirri's paws, as a kitten, and continued all her life. My childhood cat Chocolate was a scruffy kitten, and my sister's cat would jump on him, hold him down and wash him - behaviour that continued until they were rehomed separately when we left the country.
