I really couldn’t tell you how he is feeling inside his mind as we are all different in the way we process our thoughts/emotions when we enter a speech block we can’t seem to get out of easily. If the spark, banter and humour was there initially it could just be that his frustration around his stutter around you is really getting to him, and the shame of it is overshadowing the joy of the conversation in that moment. I think once he’s opened up to you about it, you’ll find things will return to normal as he’ll feel more comfortable with you once the “elephant in the room” is addressed. Just choose your moment wisely. Go somewhere nice and chill, where there’s no one around to hear the conversation, and I’m sure he will open up to you about it. You’ll both walk away feeling much better about it and it’ll strengthen your relationship. Just show patience when he’s opening up and gets stuck, which no doubt he will as it’s a very sensitive topic to talk about.