How do I stand up for my friend?

My friend has a slight problem that sometimes shows in public. Nothing serious but noticeable enough that people ask about it. The problem is the way they ask. It's not concern, it's just people being nosy or making it a thing. They come to me about it like I'm supposed to explain him or laugh along or agree that it's weird. I never know what to say in the moment so I usually just shrug it off and change the subject. But after it keeps happening I feel like I'm letting him down by not saying something more. I started thinking about why I kept going quiet in those moments. Part of it was not wanting to make it a bigger deal than it already was. Part of it was genuinely not knowing what to say without either oversharing about his situation or coming across as defensive in a way that made things awkward. I also thought about whether my friend even knew people were saying things and whether I had any responsibility to tell him. I realized that not saying anything wasn't protecting him or me. It was just the easiest thing to do in an uncomfortable moment. And easy wasn't the same as right. I didn't need a perfect response, I just needed something honest and simple that shut the conversation down without turning it into a whole thing. Talking it through helped me figure out that I didn't need to explain or defend anything. We agreed the best next step was to have one simple line ready for when it happened again. Something like "he's good, it's just how he is" and then move on. No elaborating, no apologizing for him, no laughing along. Just calm and matter of fact. And I decided to give him a heads up that people had been asking, not to make him feel bad but because he deserved to know I had his back.