I think saying he's sorry will be good. At least he is forced to admit his mistake. But since what he did has affected 3 decades of foreign policy and the image and lives of Vietnam vets, I think the only acceptable action he can make to put the issue to rest is to QUIT his run for President.
Has Kerry ever admitted a mistake? (His own, I mean -- not like blaming his speechwriters for things he said in speeches.) He wouldn't admit even to Botox. And as far as I know, he always maintained that the Fire Department just decided to move the hydrant from in front of their Beacon Hill house (which providentially opened up another parking space at the house, in an area where parking is very scarce) with no input from him.