What's the chances of maybe trying two harnesses? One worn a little tighter than the other? Of course you would need two leashes as well. It worked with one of mine when I took him out. Also, as L4H mentioned, try to leave some slack in the leash as that is how the cat manages to escape while moving backwards.
My house cats are for the most part very content to stay indoors and watch the action from the windows. One of them does occasionally want to sniff the outside so I do pick him up and take him to the deck for a few minutes. He's content with that.
Maybe what your boy would love is another kitty to keep him company. I happen to know where there are a few kittens up for adoption.
