There's some excellent advice on here & it looks like he's being treated for mites & fungal.
At the beginning of the year my girls had firstly fungal & when I thought I'd just about got shot of that, then they got mites (due to a lowered immune system with the fungal stress I think).
But Tiggy had a nasty, large (approx. 2x 50p pieces next to each other), cracked, partly raw patch on her back, the vet suggested Sudocream & this helped but it didn't seem to soak in very well & she would jump when I applied it
I did get some F10 antiseptic, barrier cream for her, which seemed to suit her better than the Sudocream. It was soothing & absorbed quicker too. Only a thin smearing was needed. Ask the vet if it's ok to use with her other current treatments.
F10 Products Germicidal Barrier Ointment
I was using Maleseb for the girls & I think it dried out their skin quite a bit, which didn't help in that respect.
With ringworm & mites the symptoms usually get worse before they get better, it's just a shame you're poor piggy has a double whammy of them
Make sure you ask the vet to prescribe the maximum safe dose of painkillers & split evenly into 2 during the day (eg, 8am & 8pm) to keep a constant supply of painkillers going through his system.
If your guineas in pain this could affect his appetite and cause other problems
Good luck finding a decent cavy savvy vet & keep us updated.
Big hug for you too as it's all very stressful seeing them like this

Xx