I don’t think he can on a whim, now that the convention is over. I am sure, if he did it, it would be done in such a way where JD was made to bow out as if it was his own decision, but good luck with that.
Any politician who is VP to an old and very out of shape old man has had more than one thought/hope about being one heartbeat away.
Also, to be real, JD very much holds up a large part of the conservative coalition, imo.