For some reason, I thought I wrote down the wrong number for the Sodium when the vet called. I thought the Sodium levels were supposed to go down as the Percorten "wears off" but this obviously went up. So I called back to confirm, and, yes, 153. So I'm a bit confused at the moment.
Either way, he's still looking good, so we're going to go another week. I'll be watching him carefully this week, just in case.