Based on the few ads that I have seen, most of Trump's ads have been focused on this topic. Biden would be wise to try to turn it to his advantage. Talk about how he had a significant stuttering challenge, worked hard, largely overcame it but still has to deal with it from time to time. How he is an advocate for others with disabilities. How he thinks we should generally support and encourage those with disabilities and not mock them (cue clip of Trump mocking the disabled reporter).
There are those who don't care that Trump mocks others. And there are unfortunately those who might think that the stutter makes Biden "unpresidential." But he has to reclaim the narrative.