I think politically to invoke 25A would be disastrous, and unsuccessful. It's not a viable option at this point. The only realistic option is for someone to really talk Joe out of running, and that is starting to look unlikely. Now, he will be under a microscope from here for any further, um, failings, his gaffes and mumbles will be covered by the media as well as his "lack of visibility", if that happens.
I don't think he's going to drop out at this point. He might have to in a couple weeks, or months, if he again looks so infirm that most Democrats turn on him.