You should know best whether such possibility exists, but make sure you don't blow that. There is no definite right answer to this, and it all depends on what's going on in the organization and the group, also personalities of your boss and boss' boss play a role.
But no matter the purpose of this meeting - 1. they want to make sure they can help your boss succeed 2. they want to make a decision to remove him 2a. promote you 2b. get someone new here, the best you can show the big boss is you are professional, possitive, and fair minded.
You are loyal to your boss, is known to everyone. So there is no need to hide nor neccessity to showcase it. You want to start off by being positive and supportive, to emphasize all the strengths your boss possesses, and of course he's very gentle and nice to everyone. You appreciate that approach very much, coz you are feeling encouraged and supported, therefore you can focus on the positives and keep driving yourself for improvements. Now, obviously same approach might be taken differently by individuals. Being nice sometimes might not be perceived as serious enough about the matter, and there were incidents you felt that he could have been more strict and dictating.
You can't say it's all great, and sing kombaya in the whole meeting. Yet you can't be too negative as someone berates his boss behind the back. You know the situation best and you should have a fair and balanced view, but present it in a possitive way.
Remember one thing, no matter what or whom the meeting is about, to you, it's always about you. Not me-me kind of self-centered conversation, but propagate your professionalism, sharp mind, and good character through the communication, when you have a chance to meet someone could affect your career.
It might be much ado about nothing, but it never hurts to showcase yourself nicely, in front of big shots, without overdoing it:)
