Well, my son is much younger but I have always talked to him about our family being like a team. "I help you, you help me, we all help each other...." He doesn't and never will get money for chores since we work as a family/team to support each other but will get an allowance someday to help him learn money management. He is almost 6.
At 9, your son could probably understand with an explanation. Maybe show him a list of all of your responsiblities from major to minor (seriously include EVERYTHING from taking care of yourself, setting the table, cleaning....)and explain that if you had help, there would be more time for fun like playing his favorite board/card game or something and that mommy would feel better and be more likely to do fun stuff.
If you still struggle, I might consider telling him no when he asks for your help with something. Tell him that your list is too long but that if he helped you with something, you could then help him. Just keep reinforcing the idea of team and that you get tired and work hard too. (Keep showing him your long list). Maybe even let him choose some things from your VERY long list that he could then put under his name that also shows things he already does to care for himself and school responsibilities. That might even boost his self-esteem to see that he is responsible and helpful.
Good luck!