Sorry, would have replied sooner, but something has been throttling our already slow net connection, all day and I tried to reply, but it timed out.
DS1 is 9, with ASD and ADHD and loves the computer. He is extremely computer literate and does more than plays games, so it is something positive for him to do. However, he finds it hard to transition from much more than an hour on the computer to something else and ends up hyperactvie and often aggressive, so we've always been strict about pacing and rationing. When I use my laptop, I might spend up to an hour, if I'm busy with something, then get up and do something else. Even DS2 can do that with the iPad, to an extent. DS1, OTOH can't, so we have to do it for him.
So, even though it sometimes creates conflict, we enforce time limits. Usually an hour twice a day, though we will extend a little if he is being productive with something. The PC he uses is Windows 8 and DH put a time control on it for a few months. 2 hours a day on non school days, 1 hour on school days (you can control it remotely for sick days, etc). It pissed him off, but he learnt to take notice and to obey the warnings it gave so he could save work. The fact that it wasn't us to argue with took away a lot of the angst. We've now removed it, but if he messes about when we ask him to save and shut down, all we need to do is ask him if he wants the timer back. It's more effective now, after the fact, than when we had it on there.
It would probably be prudent to schedule your day so there are set hours when he's allowed the computer. Make those hours less than you'd both like so there's room for earning a bit extra. If the computer OS is anything beyond windows Vista, then it should be possible to set a timer and usage monitor.