If Draco were to be confronted by a Boggart, what would it appear as to him? Was this ever suggested in the books or mentioned by JKR?
As @Slytherincess noted, there's no information in standard canon (books/JKR interviews/Pottermore).
However, the secondary canon (specifically, video games) does contain that information.
Just like 90% of other characters, Malfoy's boggart assumed the form of Voldemort, according to the game "LEGO Harry Potter: Years 1-4".
As of this writing, Draco Malfoy's Boggart is unknown.
It is not revealed in canon or in the additional information on Draco Malfoy from JKR at Pottermore. No JKR interview I have read or heard has revealed what Draco's Boggart is.
As suggested by BESW in chat, perhaps it is a ferret.
General non-canon disclaimer here. I would guess that it would be a vision of his parent's disapproval - not just his father, but his mother as well. I would imagine that it is similar to, but the opposite of, the image Harry sees in the Mirror or Erised. But Draco would see his parents either enraged at him, simply disappointed, or completely disowned.
Like many have said, there is no mention of his boggart in the books nor in the movies,however if I were to say what I thought it was (Excluding The Cursed Child) I think it would be his family (Especially his father) because he has an expectation that he needs to live up to even though he doesn't want to live up to that family legacy and he knows that no matter what his father will make him do whatever his father wants him to do because if he doesn't do what his father wants they consequences will be great. However, this would be after he realized what he was doing was bad and selfish. If we are talking before he was a good person then I think it would have to be his dad or family being disappointed in him, so again, it reverts back to his dad. In all, his dad is what he fears the most.
As a Draco Malfoy fan since day one, I don't think it's his father, I don't think it's Voldemort either, because he WANTED (at first) the glory of working for Voldemort. I also don't think it'd be disappointment, because up until Half Blood Prince, Draco is very confident, it wouldn't even occur to him that he'd ever fail and something he wanted. I think his biggest fear would simply be:
*I think he'd be heart broken to lose Scorpius after losing Astoria, but he's seen enough death to not be afraid of death. But he's always been rich, even after the war.
Losing Scorpius Hyperion Malfoy and Astoria Greengrass.