It's a nice thing that you've noticed your colleague's change and care about him. I would not recommend that you compliment him on it. Stuttering is so much more than what you hear. Inside every person with a stutter is a whole pile of emotions and thoughts, strategies to overcome or avoid and lots more. When I started a new school nobody knew i had a stutter and I managed in some way to speak fluent. But inside I had never felt worse. There's also the problem in praising fluency as the ultimate goal, instead of acceptance. I do encourage you to ask him about stuttering though. I'm always glad when people have the courage to ask. It doesn't happen too often...