This is tough. Most kids do not really like being thrown in a classroom for 6 hours. If your son is extrinsically motivated, you can try bribing. This works. But the problem is, getting them to keep working for a long time. They either tire of trying or they need so many things to work for it gets tedious.
You may try an online school. Here in the USA we have free public education online for grades K-12. It has helped a lot of kids.
Finally, if you can, you can home-school. Then the days of content are not so long.