Like other people are saying, a private conversation would be best. If the stuttering really did have a pretty sudden onset, especially in adulthood, he should seek medical evaluation. It is possible though that he has always stuttered and it was either at a lesser degree so you didn't really notice, or he may have hid it from you and has recently decided not to. Bring it up in a considerate way, but if you are concerned or just want to have a discussion about it, do it. It could be something he is struggling with.