Having looked at the question I would flag the question for moderator attention. In this scenario, as moderator, I would e-mail the OP, and explain to them how the system works, and also explain why their behaviour is not acceptable. It may sound like too much but in some cases users are worth having, and they may just need a more detailed explanation of the system. This may be one of the few cases I would consider moderators having the ability to change the accepted answer.
I would also clean up the comments and the answer a bit and set the question to CW.
Overall, the down-votes on his answer is doing enough damage, so I would not necessarily down-vote the question, but rather the answer. Your answer however is getting the right exposure since it is detailed and correct, and users are seeing the attitude of the OP.
This would be how I would handle it, however the SO moderators will use their own discretion and experience. SU/SF/SO handles certain scenarios differently because of their audience and size. This solution may not work on SO due to its size.