Absolutely necessary to switch his flea and tick protection to Nexgard, it helps quite fast and your boy will be out of trouble soon. Démodéx can be cured quite well now, but still do all these things DollysOwner recommended, as démodéx points to weak immune system, so probiotics and immune stimulators are needed to help him to beat disease. Don’t worry, it’s treatable!
If it's mange the Nexgard should cure it and you don't need any steroid injection and antibiotics. Also Nu-Stock ointment is good to rub in for mange. Sounds like the vet is also covering allergic reaction and staph infection, not sure why.