My dog worries me...help?

My dog usually doesn't go out much at all. He can only go out into the back yard so, yes he does get to see outside. But I mean we never really walk him at all. He just has to stay home all day. Im sure he would love to go outside but he can't go out running in the neighborhood cause he'll run away. And since its just too hot to walk him we don't get to walk him at all. I get worried cause he so much indoors. Especially cause its summer. Is this bad for a dogs health? My dog can stay outside and feel the fresh air in the back yard but he just chooses to come inside instead. Any ideas on what to do with him? He's a papillon about 6 years old.