I don't have kids this age and haven't been through it (yet, though I think I'm in for some challenging times already!), so I don't speak from experience.
But I agree with Nic. Say it once if he doesn't do it, he goes to school in his pj's, he goes hungry, he doesn't get the pay-off for what ever it is he's doing. How embarrasing to get to school in your pj's! I bet he'd only do it once. Then he'd do what is asked of him when you ask.
I also agreeon the no computer in the morning, or at least turn it off at what ever time will give you enough time to feed him and get him ready for school.
Tell him the new routine and be specific and explicit. The try to really stick to it. Like at 7.00 the computer goes off and mum will get you what you want for breakfast, you will eat it, then brush your
teeth and hair, pack your bag, get dressed and we'll go to school. If you don't you will go to school in your pj's/hungry blah, blah...
It's hard with kids who are unreasonable. I have one, but she's a toddler, so some unreasonableness is expected, LOL. A six yr old understands to some degree rules and expectations. If you think he needs it, I would go to the school councellor, don't be embarrassed. That's what they are there for, to listen to your trials with your children, and they'd have other resources for you to use in your area. Or maybe go to your gp if you'd prefer.Triple P have some good resources too, maybe look them up.
Good luck you sound like you're at the end of your tether.