You likely saved that kitten's life. Upper respiratory infections can be fatal to a kitten, especially one that is outdoors and malnourished. I hope you'll take the time to socialize him before taking him to a shelter. Feral kittens this young are really easy to socialize. As soon as he's feeling better, engage him in play with a wand toy and laser pointer toy. Play therapy is a great way to bond with and socialize a kitten. Spend as much time as you can with him in the garage, talking softly to him. Maybe he could become your indoor/outdoor cat? My guess is that you're going to fall in love with this little guy. He's already been in your house many times and your other cats tolerate him, so that's a great start. This kitty is lucky to have found you!