Is someone else there to supervise what he does? Nine is still quite young to expect them to work independently and be able to rely on that. Some kids will, but personality will play a big role in that. Secondly, if you are also wanting him to do his school work right after school, that can be a tough time for kids to buckle down & focus. Mental fatigue happens to us all & with kids, it's even more likely since they really do have a higher level of daily movement than most adults have. It is often very helpful to have a highly active time right after school to get energy out, reduce stress, clear the mind, before then starting into after school studies.
I have a couple of kids I watch after school (this time of year we are on break). When they come, I give them something to eat, then they are outside for 60-90 mins for play before coming in to start their studies. I have tried having them to do it right away, but it's never been great. If they have 15 mins worth of work & we start right away, that can take them an hour to complete. If instead they go run for an hour, I find they complete the 15mins in 15 mins, without all the complaints, up & down, fidgeting & laying over in the chair, etc.
Also make sure he isn't struggling with the work. A child who finds themselves frustrated, confused or stumped can often then go into avoidance & denial. They may even try to hide the fact that the work feels too hard as they don't want to look stupid. This may not be the case, but it always worth evaluating in such situations.
If you don't have anyone with him after school, then look into something like skype. When I have a tough time with my own kids (my kids are homeschooled) then I ask people I know to study with them via skype. So maybe gramma can then quiz them on something over the computer, or an uncle can talk them through fractions, or one of my "cooler" older nieces & nephews might be willing to do half an hour discussing planetary orbits. I also seek out people I know have certain interests that might be excited to "help" my kids. It is also a great way to stay connected with family & friends.