Changing the gearing on a single speed is easy, you just need one tool to remove the freewheel and then you thread the bigger/smaller freewheel you want and your own pedaling action will tighten it to the hub.

There's benefits of riding something with gears cause of course you can always lower the gearing on the fly but I still don't see how a single speed isn't a good tool to get his knees stronger. The only reason a fixed may hurt your knees is if you're pushing a huge gear and ride it brakeless but that's just my experience.