Provided that every reasonable accommodation, and even above and beyond what he deserves (which is a lot) has been made for him, if he still refuses to endorse Hillary, unfortunately I'd have to say no. It'll hurt in the GE. But it's better to have a unified voice than "a house divided." Besides, I can't remember the last time a person was given a major convention speaking slot who used their speech mainly to advertise their own causes, but wouldn't endorse the nominee. Ted Kennedy 1980?
Technically, if he forces a rollcall vote on nomination, he will be able to do a speech before the vote.