He might like something like that. You could probably find videos and plans if you search for CATIO or Jackson Galaxy on the YouTube. I'm surprised he's being such a jerk about staying inside. My formal feral bums, I mean boys, are thrilled to be inside and sleeping on upholstery instead of weeds and bushes.
If he is bored, the string toy you made is good. A laser pointer for laser tag is good for burning off energy as well.
I guess if you got desperate, you could get him one of those cat strollers and take him out on your morning walk around the neighborhood.