The theory around sudden drop in PSA is about men who are not on treatment so probably irrelevant here. The research suggests that as the cancer cells get more aggressive, they stop producing PSA because they need all their energy to multiply and spread. Let's hope that in Lisa's case, it is simply that the Enzo is doing its job.
Lisa, my reading of your latest information would be that it is getting to the time when you need to start thinking about the end. Say the things that are in your heart, tell him how much you love him, laugh about stories from years ago, don't leave yourself with anything to regret in the years to come. Perhaps your dad is not quite at the final stage of his life, in which case you will be able to tell him these things many times. But sometimes, men slip away a bit quicker than even their medical team expected - perhaps the man just gets tired of fighting and as he sleeps more and more, the opportunities to love him and talk to him disappear and before you know it, it's too late.
He could just seem so ill because of the chemo - it is a pretty aggressive treatment.