Sadly, no, I have never seen recovery when they become lethargic but my observations have been during senescence. Had he started refusing food or eating less? This is almost always the first sign of senescence, often a full month in advance of other symptoms. Their lifespans are not well studied but I would have expected this one to be with you for a full year.
I don't know of anyone using Pancur/fenbendazole in tankmate food or directly in the water with cephs so can't speak with any knowledge on its potential harm. An ammonia/nitrite spike would give symptoms but if it had not continued over a period of days I would not expect this lethargy. However, I would not discount the dewormer as the direct culprit. I used it (long ago) in a seahorse tank and lost a serpent star even after a full water and bottom substrate change (it tends to be absorbed into the rock and then leach out - partially why it is used for worm control in aquariums). I do use heartworm and flea poisons on my dogs but avoid any kind of poison control in or around the fish tanks (no aerosols at all) because cephs are so sensitive.
My only thought would be to move him to a bare QT with newly made water and see if there is any improvement.
Yes, being touched (mentally and/or physically by an octopus has a strong effect. If you have not done so, I will suggest Sy Mongomery's, Soul of an Octopus. (I highly recommend the book but if you prefer short stories/articles, This article and this article (written several years before the book) will give you a feel for the book (reading the articles will not be a complete spoiler for the book) .
I don't know of anyone using Pancur/fenbendazole in tankmate food or directly in the water with cephs so can't speak with any knowledge on its potential harm. An ammonia/nitrite spike would give symptoms but if it had not continued over a period of days I would not expect this lethargy. However, I would not discount the dewormer as the direct culprit. I used it (long ago) in a seahorse tank and lost a serpent star even after a full water and bottom substrate change (it tends to be absorbed into the rock and then leach out - partially why it is used for worm control in aquariums). I do use heartworm and flea poisons on my dogs but avoid any kind of poison control in or around the fish tanks (no aerosols at all) because cephs are so sensitive.
My only thought would be to move him to a bare QT with newly made water and see if there is any improvement.
Yes, being touched (mentally and/or physically by an octopus has a strong effect. If you have not done so, I will suggest Sy Mongomery's, Soul of an Octopus. (I highly recommend the book but if you prefer short stories/articles, This article and this article (written several years before the book) will give you a feel for the book (reading the articles will not be a complete spoiler for the book) .
I have noticed a decrease in appetite over the last couple of weeks, but that also is roughly the same timeframe as when I used to dewormer. I wasn't too worried because although there was a decrease, s/he was still taking food every few days. 
