It’s really hard to dismiss someone in a medium size enterprise with its own hr just as the boss doesn’t like him.
I think you’re reticent to accept he’s on a pip for a reason, but he’s telling you there are improvement measures. He’s fully accepting that, and telling you he’s meeting these improvement measures. If he was someone who did the work of others, regularly got his boss out the shit, then it’s unlikely his boss wouldn’t like him and they would be unable to come up with improvement measures.
i think it’s clear you love him and beleive what he tells you about how much of a star he is, he’s positioned himself to you as the department saviour. But you also need to balance this with a man on pip, with clearly defined improvement measures. Improvement measures he accepts.
if he’s being managed out due to performance, I suspect this meeting is an initial discussion telling him if it doesn’t improve it will move to termination. At that point there may not be a settlement.
you will need to wait to see what happens at the meeting, but I’d also consider he may postion himself as fab again and not give you the truth, likely as he won’t want to disappoint you.
if that’s the case and if eventually moves to termination, he needs to negotiate to resign and a reference, otherwise he’s right, he won’t find another job easily.