"He reads very badly."
Translation:Er liest sehr schlecht.
"sehr" is the normal translation for "very".
"ganz schlecht" is quite informal and wouldn't be used in written language. It's used to emphasise that something is "very / really bad": "Ich kann ganz schlecht zeichnen!" = "My drawing skills are really bad!" Often it doesn't sound very elegant.
(A phrase you might come across is: "Do you have a minute?" "Nein, es ist gerade ganz schlecht." = "This is a really inconvenient time.")
"ganz gut", on the other hand, works much better: "Wie geht es dir?" "Oh, ganz gut." = "I'm doing reasonably fine."; or: "Das Bild ist dir ganz gut gelungen." = "You did a reasonably good job / fairly well with that painting." - which is not too much of a compliment.
"Das Bild ist dir sehr gut gelungen", on the other hand, means that you did a great job.
In spoken language, however, the meaning depends on the emphasis: "Er ist ein ganz berühmter Künstler" = "He is a really famous artist", but "Er ist ein ganz berühmter Künstler" without the emphasis = "He is a fairly famous artist".
Sometimes "ganz" seems to have a connotation of surprise or worry:
"Er liest ganz schlecht" = either "He's really bad at reading" (with an emphasis on "ganz"), or (emphasis more on "liest") "What's he doing? I thought he was great at reading, but he's not doing it at all well now!" / "I just found out he really lacks reading skills! We've got to get him some tuition!"
"Sie ist ganz hübsch/nett" = "She's fairly pretty/nice" - this is either unenthusiastic ("well, she's not ugly...") or a bit surprised that she's "quite pretty/nice (after all)!".
"Du bist ja ganz blass!" = "You're / You've gone all pale [in the face]!"
"Du bist sehr blass." = "You're very pale (likely because that's just your skin tone)."