Personally, I love when people ask about it. I think it's important that people ask their serious questions if and when they're curious about anything. My friends often ask me things about it and I answer. I don't know if they feel uncomfortable asking but I've never felt uncomfortable or offended answering questions. That being said, if you have any doubts maybe ask if you can ask. Like start a conversation dedicated to your questions and curiosities. Ask if he would mind if you ask him a few questions about his stammer. Follow his lead, if he's uncomfortable then that's a no, if he's uncomfortable or enthusiastic then have at it. And of course if he says no, you have all of us to fall back on for the basics.