First of all, you should be proud that you took the interview and that you received positive feedback from the interviewer. I don’t think that you bombed the interview if that was the case. Does the role require you to speak often? If you want you can always email the interviewer and disclose that you stutter. It's a good idea to send a follow-up email anyway so you could just add that at the bottom. Anyway, good job and good luck!