Agreed. My own mother (who ironically I think I might’ve gotten the stutter from genetically) has done the same thing as OP’s BF to me before on numerous occasions in the past. So has my sister... I know they were joking but the it’s the principle. It definitely made me feel horrible and I think it’s so disrespectful considering it’s something we can’t control and are born with. But honestly I never said anything or called them out on it but it hasn’t happened in a while (few years) and my stutter isnt that bad. I think it’s more than rude though and as I’ve gotten older I’d reconsider or at least question the ties I have with anyone who does some shit like this with malice.