Well i mean if you feel that he is comfortable to talk with you about his stutter then choose option a which overall is the better option because you will talk things through and will help his confidence to grow.But if you think that he will not like it i mean if you guys are not that close then stick with option b until you think he is ready for this kind of conversation because if he is not it could embarrass him.But overall option a) is the best one you have to just choose the right time.