I would keep toys around the house while you are gone. Our cats love to play while we are gone.

I know this may sound dumb, but ever since we got a fish tank, Noah spends most of his time during the day on our desk chair staring at the fish swimming around.

The bird feeder is GREAT! Noel spends almost the whole day staring outside, chirping at the birds.

I'm not sure if its an option for you now or in the future, but another cat would help A LOT! I am greeted at the door daily by Basie and Micah, so I know they are playing during the day as well.

I don't remember how old Boo is, but cats sleep a lot during the day, even if they are outdoors. As far as keeping him fit, make sure to limit his food to his recommended amount.

Hope that helps.