Tell him! The sooner, the better. If you don't know how to do it, start with telling him that you thought this thing was insignificant to mention, that's why you did not tell him before. Also tell him, that you have good and bad days in terms of how strong your stutter is, so he will be prepared for that, because maybe you will start to stutter more often in front of him, but it is better than living your whole life hiding it from him.