My daughter has been getting about 15 pieces of homework per week!! She spends varying amounts on each piece, maths takes about 15- 20 minutes as she is very fast. Written pieces take her much longer as she is dyspraxic and dyslexic. Have to say that I was worried at her spending well over an hour on one piece of literacy only to learn today that she has been given a merit for outstanding acheivement on that particular piece of work
Moral is I guess - you only get out of something what you put in, but you have to want to put in the effort. I know I would do the same in your position and present extra work and I don't think my daughter would mind. My son however is totally different and judging by the info we were given at the induction evening at our secondary school a lot of boys do
adopt a more laid back approach
Have you asked him why he doesn't want to do a bit extra, is it a possibility that he's worried that its not cool to be seen to be clever? Perhaps you could persuade him to treat it as ongoing revision to avoid masses of cramming at exam time.