I thought maybe it was the heat, but I took him out at 6am the other morning and he still hated it. We drove near a highway once, the sound of which set him off, but I've been taking him to quieter streets and it makes no difference.
It sucks, because this was part of my plans for summer touring and riding. I mean I wasn't sure if he could handle long term but I was planning on taking him along for some trips closer to the area at least.