[ Guys , would you know why his shell is like this?]
Directly tied to its habitat. A photo of habitat will help a lot.
Read here for treatment :
http://www.redearslider.com/shell_conditions.htmlLittle young but your little guy is a girl. Good photo of cloaca. The cloaca will not change :
http://www.redearslider.com/determining_sex.htmlAlso is this one of the two turtles you got back in January that also got an infection ? Reread treatment then , will be the same. Separation a must very contagious !
This can be stopped once you correct there habitat.