Mine ds age nearly 9 is the same. For him, it's largely down to his level of interest (like with your ds' computer games) - if a subject is attractive to him he'll focus on it for hours if he's allowed but being forced to do something he regards as pointless is torture for him. We use reward strategies too with mixed results - the most productive is a family movie night on SAturdays if all the homework is done by then, and we take turns to choose the movie, have favourite food for tea etc.
For J there's a considerable difference between types of homework. He's very advanced at maths so finds that easy, therefore it's less effort to get him to do it and the reward incentive is more likely to work. However literacy is a real problem for him as he finds it so difficult to think abstractly, eg questions such as 'How do you think such-and-such character felt when...?' - he's never been in that position before so can't relate to it, so can't answer the question as it's not in his scope of experience. So a lot of it depends on whether or not he is actually capable of the task and if not the incentive will make no difference.
Are your son's teachers aware of the difficulty he's having producing work? Perhaps they can suggest some strategies that you haven't already tried. It's worth asking, but I suspect you're probably doing all they'd advise.
A lot of children with AS have a very definite distinction between home and school and can't accept doing schoolwork at home. School is for work and home is for relaxation or whatever. It's rarely a conscious choice but a difficult problem to deal with nonetheless.
Fish oils and diet do make a difference for a lot of children but even with all those interventions in place for several years, J still has huge difficulties focusing on homework.
But in answer to your OP, what you're saying is very common in AS. I'd say more children have problems with homework than not.