If possible, I would ask for a meeting with the people who provide therapeutic services for your son. They are in the best position to give you advice on how to handle the homework situation. One idea I have is that these children often need a very well-understood structure that is followed both at school, during special services and home. For example--we will work for 20 minutes, then we will take a 5 minute break when we get a drink of water and a snack, then we will work for 15 more minutes. This is planned out ahead of time and the child knows what to expect. Many children also respond to rewards such as getting to do something fun at the end of the homework time.