You're speaking with Dr. Michael Salkin. Welcome to JustAnswer. I'm currently typing up my reply.
Sorry for the delay. I'm visiting relatives in North Carolina. To determine if the pigmentary change is significant I would have to skin scrape looking for the rare Demodex mange mite, and culture looking for bacterial (Staph, mainly) or fungal infection (unlikely in a purely indoor cat). More likely, the pigment represents a benign idiopathic (unknown cause) change at Chumley's age.
The Convenia is ideal for a Staph infection and one injection might suffice. What I can't know, however, is if the hyperpigmentation represents a Staph infection. If he were shaved and you saw roughly circular lesions (epidermal collarettes), a Staph infection would be most likely.
I agree. A Staph infection is likely with that type of antibiotic inside him.