I also appreciate all these replies.
BJM - Clyde is only 3.5 so it seems unlikely to be CDS.
Dr. Schrodinger - sadly, there aren't any options for increasing the territory as both my cat sitter (and I) live in one room cabins. We also live in Alaska, so even if the cats did go outside (they don't) they wouldn't be able to now as it is still far too cold. There are lots of windows to look out and birds to watch, but I don't really know what's causing his discomfort, other than just being in new surroundings.
I have been trying to reach Clyde's vet to ask about increasing the phenobarb dose. When he was displaying a little of this at home (to a MUCH lesser degree), she suggested that he could be having tiny seizures - like smaller than even petit mal and was disoriented. But she didn't suggest at that time that we alter his dosage. But I do hope to talk to her tomorrow, since the behavior has become far worse since being at the cat sitters' house.
rvontrapp - I don't know if my cat sitter has found anything to work yet. I had to have the Feliway sent to her and I think it is due to arrive today. She lives in a remote area with no access to any stores. I'm doubtful that there will be any immediate effect, but I needed to at least try! I did suggest the extra attention/play. She also has switched from giving him the oral meds directly to mixing it in his food, so her only direct interactions with him is loving and not stressful. He doesn't get stressed at all when we pick him up and put the meds right in his mouth, but he was starting to hide from her and fight it a bit.
So, that's where we are on that. I may have gotten better b/c I haven't heard anything about it in the past two days. Either that or my friend is just being really polite to not bring it up anymore. I hope its the former!!