My school secretary offers to give me rides home on rainy days or dark evenings (I'm a good forty minute round trip out of her way, but she is serious), out of concern for my safety. It sounds as if the OP's boss is similarly well intentioned, albeit a bit over the line. I have taken to answering as if it was out of concern from getting damp and mention my cool new rain paints or Thoreau's quote about wet and cold being as important as warm and dry, or yes, it IS dark out isn't in, haven't you seen the beautiful crescent moon yesterday... I saw it riding home over a hay field with a bit of mist... did you see Jupiter rising, man it'll be good to have those bright winter constellations back again....Later, I might mentioning that my bp has gotten more reasonable when I've given blood (something she and I had shared in the past as a common concern) among other less personal examples helps make those points without arguing them. By example and enthusiasm I hope I point out how the benefits of riding outweigh risk.