I'm not sure exactly what you are asking, but I'll give you the best one I've got. Being overweight doesn't have anything to do with being responsible and doing your homework. Good study habits are learned, so as a parent, you have to monitor your children carefully. He might be haivng problems with the math and making excuses so he doesn't have to do it.
Now, there is another issue. If your son is obese, he stands an increased chance of bing diabetic. Having undiagnosed diabetes could affect how he does in school. Higher blood sugar could affect his concentration level and leave him with very low energy most of the time. Your fist step would be to get him to the doctor so they can check him for diabetes. Then, if he is fine, you need to set down homework rules, like no tv, computer or any electronic devices until homework is done, or none of those things if he doesn't bring home all his homework each night. You can contact his teachers and tell them about this problem. They can send you a letter telling you what assignments are due and when and she can ensure that the books he needs at home come come with him each day. You need to put effort into this.