OK, following advice here and thinking about what would be best for us we stopped using Khan Academy for now and are using a workbook from Critical Thinking Company that is challenging for DS, but that he can do on his own... and that I can do too!
In case anyone is interested it's Balance Math and More Level 3, which says it's for grades 6-12. I allow DS to use a calculator to do the questions because it is more about logic and algebraic reasoning than calculations, which is perfect for both of us (me because calculations are my bane and DS because long division is his bane). We both find the problems to be fun to solve and I'm feeling much less stupid!
Unfortunately he's going through it pretty quickly and there is no harder level, so soon I will have to find something else to replace it. Maybe between this and Beast Academy guide books and rereading Penrose and the occasional math video on youtube we can stretch our current materials enough to cover the summer break!
Overall I have to say, thanks so much guys, problem solved
