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One child should not be left to educate the younger sibling. Not sure why your husband wants that (he can teach your older one responsibilities in a different way if that is what his goal is) It would cause resentment to both siblings otherwise. What if the younger one does not perform well? Should the older one be held responsible for that then?
It is ok for siblings to help each other and there should be some limits to it (healthy limits) Perhaps your husband does not see possible problems that can arise from this.
If he brings up the issue, don't ignore it and try to remain calm. Make a mental outline of the cons if this is the set up. The younger child should look up to an adult to learn the math not his brother. There are educators and then there are siblings. The 9 yol has to start to learn independence and good study habits. Entrusting a sibling as his mentor may not really be the best.