In many fanfics, Draco's reaction turns into a sense of protectiveness. He feels a need to shield Harry from any further harm, whether it's from the people who abused him in the past or from the judgment of others. For example, in some stories, he'll stand up to those who make snide remarks about Harry's past, using his Slytherin cunning to put them in their place. He also tries to be a source of comfort for Harry, which is a big change from their initially antagonistic relationship.
Draco might initially be shocked. He could then feel a sense of protectiveness towards Harry. Maybe he starts to see Harry in a new light, not as his rival but as someone who needs help.
There's also 'Hidden Scars' which shows Draco slowly realizing the extent of Harry's abuse. The story is really good at portraying Draco's shock and his determination to be there for Harry. It's a very emotional read with a great character development arc for both of them.
One possible plot development could be that Draco tries to expose the abuser. He might gather evidence secretly and present it to the right authorities at Hogwarts.
Draco's reaction could be complex. He has been brought up in a certain environment where showing weakness is not acceptable. But when he discovers Harry is abused, he may find it difficult to ignore. He might start by being a bit standoffish as he processes this new information. However, deep down, he might feel a growing empathy for Harry. His actions could range from small gestures of kindness, like leaving a potion that could help heal Harry's wounds secretly, to gradually becoming more open about his concern and trying to get Harry away from the source of abuse. This discovery could be a turning point in their relationship, leading to a deeper understanding and connection between them.
In the fanfic 'Malfoy's Revelation', Malfoy is paired with Harry for a project. As they spend time together, Malfoy notices some strange behaviors from Harry, like flinching at sudden movements. After some snooping around, he discovers that Harry has been abused. This new knowledge makes Malfoy more empathetic towards Harry and he tries to help him in his own way, which includes standing up for Harry against some of their mutual enemies at Hogwarts.
Harry's reaction would be complex. He has been at odds with Draco for so long that this revelation completely changes the dynamic. He would likely experience a sense of disbelief that such a thing could be happening to Draco. As he processes it, he would start to feel empathy towards Draco. Harry might approach Draco cautiously at first, not sure how Draco would respond to his newfound knowledge. But he would be determined to help in some way, whether it's by using his influence in the wizarding world or just being there for Draco as a friend. This could also lead to a significant shift in the relationship between the Gryffindors and Slytherins as a whole, as Harry's actions towards Draco could inspire others to look at Slytherins in a different way, seeing that they too can be victims of difficult situations.
One popular fanfiction with this plot is 'The Unseen Bond'. In it, Draco's discovery of Harry's abuse leads to a complex relationship development. Another is 'Hidden Scars', where Draco becomes Harry's unexpected protector. There's also 'A New Alliance' which focuses on how the wizarding community reacts to Draco's new - found knowledge about Harry's situation.
One possible reason could be his family's status in the wizarding world. His family might be involved in some dark deeds, and Draco could be a target because of that. Another reason could be related to his own actions or attitudes that have made him enemies.
He may start to spy on Ron to gather more evidence of the abuse. Draco is cunning, and he would want to be sure of what he's dealing with. Then, depending on what he finds, he could either take matters into his own hands or inform the proper authorities at Hogwarts, like the teachers. He would want to be in control of the situation in some way.