Do you think giving him some sense of "ownership" might make him take better care of it, or does he trash his own stuff too?
Take it apart (not too much), tell him it is broken but can be easily fixed if he wants to keep it afterwards, then invite him round to help you "fix" it. Without being obvious, get him to do most of the work, give him some positive encouragement as he puts it back together, so that when it is "fixed" he's got a sense of achievement and feels it is more his than just any old bike to be chucked around and abused?
You know him better than we do, so it might not work and he might just be one of those kids who doesn't respect stuff, but if you're thinking about letting him have it anyway, it could be worth a try.