You are a good work colleague! Everyone is different but in the past I would have wanted my work colleagues to ask me if there is anything they can do to help and also for them to ask if I wanted them to help when I was completely blocked. Even if they got the wrong work it broke the cycle of blocking and helped me move on. It's a funny one, you know he stutters, he knows he stutters and yet it's not spoken about. It's hard to know if he is open to talking about it or not but you sound like a very caring colleague so maybe maybe start by asking if he would like any help at all. Good luck.