It is nice that you are backing up the school, but at this age punishing at home for something that happened at school really doesn't work. Yes, he definitely needs to get his homework done. But basically, kids of this age need immediate cause and effect. To punish them for something that happened hours or days ago, really doesn't connect very well. Maybe about 3rd or 4th grade your tactic will work, but not now. The school is the one that needs to deal with this. The way they do that is to figure out what is going on. They should communicate this to you and what they are doing about it. If they expect you to deal with the situation, then they are avoiding the problem (if it is school based). Which makes benjimom's comments very important. If they can give you certain behaviors to reinforce at home, that would be very helpful.
I would ask him if something happened at school, esp since his behavior has changed. Has something also changed in his life, any major life changes, etc.....