My cat is lethargic with a face sore. Should I take him to the vet?

I hate to hear that Timmay is not feeling like himself. I would recommend to get Timmay seen by his veterinarian today if possible given that he has been lethargic for a week. They will check his temperature and give him a physical exam. Lethargy can be due to fever, organ dysfunction, anemia, heart disease, infection as well as other causes. That scab on his face could be a few things. An abscess could present like this and it could be superficial or related to a tooth root abscess. Superficial bacterial infections, thyroid problems or allergies could also look this way. The vet will try to figure out the cause and will prescribe appropriate medications.
Given that Timmay is 18 years old, the vet may recommend bloodwork to check his enzyme levels and bone marrow function. Older cats are more prone to kidney disease as well as other problems with organ dysfunction.
You will want to keep the area protected with an elizabethan collar so that Timmay cannot further cause damage it.
I hope this helps and thank you for using Petco Pet Education Center, formerly Petcoach!