I am a doctor, and, believe me, having gone through years and years of schooling, and taken many standardized tests - an 89 in the 7th grade is a drop in the bucket. It is not a big deal, it just isn't. Even the Honor Roll - while nice to be on it, nice to brag about it when you are the parent - in middle school is also not a big deal. It's not like you can put it on your resume when you turn 21. I say, take it easy on your kid. Praise him for his efforts, first and foremost. Ask him how he feels about his grades - he may have already realized he should have worked harder, and, if this is the case, there is no reason to ground him since he knows where he's gone astray already. Then, you can, if you would like, push him to try harder. Personally, if it was my kid, I would not - there are more things in this world to talk about than missing the Honor Roll in middle school by less than a point.
If it helps, I missed being valedictorian by less than a point. Thankfully, my parents did not care, and neither did I, and now, no one cares. Life goes on.