I would say it's too much. After being in school all day - what happened to childhood? Why is a 6 year old doing extra work after being in school, forced to learn about things they're mostly not interested in (just to tick government boxes) all day, day after day?
Relax and let him enjoy his childhood! Children learn naturally. Turn them into mini regimented workers too soon and you will likely squash all love of learning at a very young age. The fact you're having to battle to get him to do tasks now suggests he's already fed up. Fed up and bored - of learning. Of working. At age 6. That's incredibly sad.
The positive is - he's young enough to turn it around. Try asking him what he's interested in. Any topic at all. You can have fun and incorporate maths, English, science into that given topic, without him even realising.
Find what he is interested in and you will find he'll learn more in a 30 minute session than he will in an entire week, being force fed information at school. And the best part - it leaves him thirsty for more.
I fail to understand why we have a society (in general) of pushy parents who think they are doing best by their children by forcing them to sit and write essays. To learn about things that, to them, have no relevance. And the truth is - even as adults, the majority of things learned in schools, really do not have any relevance to our every day lives. The children are right. And to think that pushing them so hard at such a young age is going to put them into a high paying job as an adult is also very misguided.
I guess it depends on what your idea of success is. If money is success to you - then by all means, risk your childrens future happiness for the sake of having them memorise enough facts to pass an exam. But that's still no guarantee of more money in adult life. If your idea of success is to produce confident, happy, well rounded adults - then back off and allow them to be children, to maintain the wonder and excitement, to enjoy learning and let them find their own way.