Well it's good news to hear that tetra diesease is off the table, sorry for showing incompetence.
I wish Colin was here too for your sake, I simply don't have the experience to help, but I couldn't just sit on the sidelines and read the thread, I had to offer my hand in one way or another.
The redness could well be a sign of septicimia, but If it is septicimia I've heard that is quite hard to treat, alot of cases the symptoms shown normally mean it's in the advanced stages. Though the question should also be what caused this in your fish?
There are many treatments for septicimia so I'll list them, though I'm sure you've already looked all this up yourself. (just a note I've C+P'd this)
I wish I was more experienced for your sake, I'm just hoping that someone pops in with some valuable information for you.
Just a disclaimer, in no way am I saying this is septicimia, I just thought that what information I did dig up would be better served being posted rather then kept in my head. I'm sorry about the condition of your little guy, I hope it can be resolved.