Word like "decadence" for declining skill
Decline was the first word that came to me, too.
If you're looking for a word to describe his funk itself, though, what about something like indifference or lassitude? Even malaise might fit.
These aren't synonyms of decadence, of course, but when I read your sentence, decadence isn't the word that jumps out at me.
As alternatives, my ‘Collins Thesaurus’ gives degeneration, decline, corruption, fall, decay, deterioration, dissolution, perversion, dissipation, debasement and retrogression. But then again, perhaps you don’t mean decadence at all.