It's good he's gaining weight at least. You can probably expect those blistered areas to slough off with the next shed. May end up having some open areas bc of it: I forget, but you have him on paper towels right? You definitely don't want debris getting into those places once the skin comes off.

Have you told the vet the infection is getting worse? It could be resistant to whatever meds they gave you. With the shape this little guy is in I'd go back and ask them to do a culture (unless they already did one). The way its over his whole body evenly makes me feel like it's something systemic rather than just a localized surface infection, so your time may be limited if whatever this is happens to be resistant to the meds he's getting.

Best wishes for the little guy.