I see this as a 2 part issue. 1) the use of technology and 2) the way he handles not getting what he wants (bending rules, complaining etc)

Does he behave the same way with other things he wants to do? Or is his complaining purely connected to use of technology?

Looking at this a different way. My DH is gamer and has an addictive personality, if he did not have gaming/internet, there would something else he was really focused on. Also my DD1 is a complainer with anything that does not go her way, so watching TV, computer, Wii, food she wants to eat, going home (the list goes on). So living with them, I can see some of what you are going through.

I agree that a timer would frustrating, would be more inclined to be task based. "when you have finished that level/game/etc" then you need to get off and do X". The more he complains, the less time/warning/confiscation etc. The more compliant, then a bit more time, so gets to play another level etc.
Other than normal kid complaining for time to time, it's only around technology. It's the only real issue we have, which is maybe another reason I'm limiting as I know he's addicted, I wouldn't send an alcoholic into a bottle shop and tell them to do whatever they like lol.

A lot of the games aren't level based so that doesn't work (we had considered it before)

We tell him when there's 10-15 mins to go but that still isn't "enough" when the time is up.

The kids are already responsible for the consequences to their actions, we don't really set punishments. We discuss with them what they feel is appropriate.