Ask him if he'd tell someone in a wheelchair to hurry up. Tell him that stutter is a disability. We wouldn't tell Jane in a wheelchair to hurry up. "Jane, you're taking forever to get on the bus and out of the bus!" We wouldn't attribute anxiety as the cause of her condition. "Jane, if you'd just snap out of your social anxiety, you'd be able to get on the bus without taking forever." We would not say things like "Jane, I noticed that every time you have to go somewhere, you do this weird fidgeting thing, like, you spin those big wheels. My nephew is autistic. He used to spin the, whatever is that spinning toy thing, all the time. And he doesn't do that anymore. If he can stop, you can too."