Give him something to work for such as a sweet, a toy, etc. Tell him if he does his homework nicely he can have a lolly. Stay with him the first few times and have the sweet in your hand, pocket, or in a jar somewhere visible. Once he understands the method, you can keep the sweets in the pantry and leave him to his homework. Good luck!

I think you need to spring him to a place have some children. In this place, other children will do tasks of them. When your son in this enviroment, i think he will become more responsible!

Hello,

Motivating kids to get their work done can be difficult.  I have a M.S. in ABA (Applied Behavior Analysis & Autism) and have worked in the special education field for six years, and I still struggle with this on a daily basis.  It's hard to say what the specific reason(s) for your son's work avoidance could be.  It could be as simple as the fact that he'd rather be doing something else (playing!).  It doesn't sound like the work is too difficult or that he is having trouble grasping the concepts, because as you said he is able to display adequate work at other times.  

For me, I'm always trying to find different ways of motivating the students I work with.  This changes constantly, from tangible items and edibles to specific free time activities and other privileges.   It may also be worth while to explore a reinforcement schedule; I have found that some children I have worked with with autism need some form of reinforcement more often than other children with emotional disturbance who can tolerate more delayed reinforcers.  I realize that your child is probably typically-developing, but using a reinforcement schedule can be helpful for many children both with and without special needs.

I'm hoping that some of these resources can be helpful for you, and wish you the best of luck with your son!

you could give him his favorate  food/treat.