The issue isn’t the rash itself, it’s that he’s scratching it in his sleep. We’ve seen him break his skin at t once, that’s a great way to get an infection. Failing vision is a serious indicator of sepsis, he should definitely see a doctor and needs to be hospitalized if that’s what it is. The vision problem could also be from stress though.
Edit: It will likely take some willing suspension of disbelief if it is indeed sepsis due to a lack of other common indicators.