Superboy gained that t-shirt/jeans combo when he started living in Smallville with Superman's parents. This is also when he gained the name 'Connor Kent' and stewardship of Krypto.
It's rather low-key and fits with the setting. He does farmwork now, Adventure Comics points out that one of his available classes is Animal Husbandry, and his plainclothes is a flannel shirt over top and glasses, both of which are easily removed when he needs to.
He did wear the classic costume during his stint with the Zero Hour Legion, but otherwise forgoes it because he's not Clark, nor does he really feel worthy to walk in Superman's shoes.
Superboy costumes in order:
The original
The Mid-Range
Number #3
Legion Stint
The Current Outfit
While I don't like Wondergirl in the t-**** and jeans, I do feel it works for Superboy rather well.
I assume Keith that you're just catching up with Adventure Comics. Just so you know, this Superboy is clone created from the DNA of both Superman and Lex Luthor.