Mike Hucklebuck?
My stupid brother was all for Huckabee before he jumped on the Trump celebrity band wagon. I have no doubt he would wag his tail back to Hucklebee. ~sigh~.
He does say that polls show a lot of overlap between Bush and Trump supporters. Those are the two best known candidates so it's not really surprising. If you've only heard of two or three candidates, your second choice if your guy drops out isn't going to be somebody you never heard of.
Trump voters don't fit into any narrow group. They won't all go to the same second choice if Trump leaves the race. They'll find as much to dislike as like in Jeb or Huck or Ted or Chris. Given his high poll numbers, Trump will still be around when a lot of the other candidates have packed up and gone home.