I'm so glad this post has helped someone else, too!

I'm going to go ahead and put it in the water downstairs for the downstairs cats. I had to clean the fountain anyway, so I just went ahead and made up a batch. I figure it won't hurt them. And I am doubling Butters' doses upstairs. And I also am thinking of getting another fountain, because I need the filters anyway, and ours is black (which shows the water splash droplets, and I'm a crazy person and that bothers me). That has nothing to do with anything.
Anyone know how long he should be separated? I'm no vet, and I'm new to this, but I just don't understand why he NEEDS to be if we really think it's herpes. The other cats probably already have it, or if they don't, they will in time anyway. There's a chance it's NOT, but I think since it didn't respond to any of the antibiotics, it probably IS.
AND speaking of antibiotics, is there anything I can do to help him? He's not even halfway through his 10-day dose, and I can tell it's giving him the poopies.

He's eating and drinking plenty, so I think he's ok for now as far as being dehydrated from that, but I'm a little concerned nonetheless.
The vet wanted to keep him separated for 3-4 weeks (which would be about the first of April), but I'm really wanting him to get down there and get to know his new house. I've held him in my arms and let the other two cats see him, and there were no hisses or growls. Just curious kitties. Both of my two downstairs babies seem to be social, so I think everyone will get along.