Husbands credit / need quick-ish fix

Hi. Husband had a score in the 700s and it dropped to 600s last month. He had an account that was posted as a loan closed out derogatorily. It was charged off/written off and that impacted his score. Any tips on how to get this disputed or whatnot quickly? We’re looking to buy a house soon and they’ll use his credit because it’s the lowest. Thanks for any help!