To me it sounds like you're doing just fine! We totally have moments like this with DS. I am at my wits end on what do with him for science. He's not ready for the work load or rigor of high school science. But elementary science seems so light in general and just generally obvious for a very GT kid.
One thing that I do that keeps ME sane is to have "office hours" and "school hours". From get up to 3 pm is school hours (no video games or TV). But I only teaching 2-3 hours of that time. I cannot teach and engage all day long. When we're home, my kids usually have at least a few hours a day where they need to invent their own entertainment or education. You also might want to just let him go nuts at the library on a weekly basis and pick and choose what he wants to read and research. My son does a lot of internet digging and research too.
Good luck and hang in there!
