If you have read my previous post on homework, I suggested setting aside a part of everyday where you set up the routine to teach your child to take time accomplish something independently.
For all children, and especially children with special needs, working independently is a vital skill. WE all need to be able to work on something alone. That could be a review, work on incomplete work, read or listen, draw or write, do a math problem, practise a math skill, or read up on a science topic. Over the year, this will become second nature, and become a time that is just part of the routine!
Set up a place and time every day for this to happen. No extraneous noise or distractions. Use the visual timer and make it a short time. You might want to sit close by, but do not interact until the time is up. Even if there is nothing accomplished, you will see a change in behavior over time. You can even model what is expected by reading, writing, or working on a problem quietly. Don't let anything short of an emergency interfere with this time.
For instance, start with 5 minutes--and gradually build up based on the age of the child. For older children, build in a break after 20-30 minutes, but insist on getting back to the time you think is needed. Two 30 minute focused time periods is much better than 1 hour of wasted time.
Make sure your child has a good preferred activity to go to after the structured "homework" time. Once school starts, homework may be assigned. If it isn't, then insist on this short period of 'homework' time for quiet work anyway--reading, writing, math skills, letter writing, drawing, planning a lego project!