Since the beginning of our relationship my boyfriend has always told me he finds it cute. He's the only person that can say so without me feeling dragged down, cause I know he understands what stuttering means but overall doesn't feel like that defines me. Should any other person told me it's “cute” I'd probably feel mad as hell, it feels like a nightmare when trying to speak, nothing not even remotely alike to pleasant or cute. So I guess if you tell him just apologize before hand telling him you meant no offense, in case he doesn't like that. I'm sure he won't get angry if it's carefully coming from someone they love.