I try to clarify my moods for my H. Only because I can see he is just like me in that when he tries to read my mind he assumes the very worst. That is up to you though. Further, maybe you should ask him to help with HW on scout days if he is already going to be there. If he continues to not help him then you have an issue. You might just say something like, "I'm sure S would like you to help with HW on scout days. By the time we get home he is very tired. It seems to take longer and he gets frustrated more easily." so it doesn't sound like you are blaming but instead states honestly what the issue is and how he can help it.
