Our instincts can lie to us but as a social species we should trust them. Why wouldn’t it be his stutter? Stuttering is associated with only negative traits by the public at large. I’ve been told to my face I couldn’t be hired because of my stuttering but that was 12 years ago. People are not going to admit to discrimination anymore and always deny. Stuttering has a drastic effect on first impressions. I’m not trying to be negative, I am trying to be validating to what OP is going through. I’ve been through many similar situations.