If I may suggest so, many months late, perhaps Harry is snooping (as usual) and finds the sweater. Knowing Severus never wears green he wonders if it's for him and if so why it's so old. Being Harry he tries it on, it fits well enough and is comfortable. He falls asleep in it while waiting for Severus to return home. Severus sees him wearing it, is furious beyond words and slams the door, waking Harry. He's about to snarl about foolish, snooping Gryffindors but the words die on his lips when he sees how much the sweater suits Harry; brings out his eyes, fits his skin tone, complements his dark hair and hangs just right off his shoulders. And suddenly he realizes he still loves Reg but he's finally truly over him.
Or something.
Sorry, that just popped into my head after reading your post. Shutting up now.