This is what I would do with the situation you describe if it was happening with DS. (This is to give you ideas, not claim it as the right way).
I would forget about the 'things' and try and define the behaviour. The behaviour is that ds becomes aggressive when being asked to stop a highly preferred activity.
Then there are few things I might try.
One is: Remove the highly preferred activity completely for a while. Introduce a preferred activity that isn't so 'addictive' and practice short slots of the activity being stopped with a reward of a highly motivational toy/piece of food/praise, whatever. So what you are practising first, is him stopping an activity when you say so, of something he isn't that bothered about stopping, and then when you have that level of 'control' you can increase the level of preference, finally introducing back the phone.
Another way is to simply start with the phone, and practice stopping him from using it purely for the practice when it matters much less whether he does or not (that will reduce the anxiety on you as you are just practising the skill, not trying to get him to learn/complete the skill at the same time as not be late for school etc.). So let him have it. Tell him it is for 5 minutes only. After 5 minutes remove it. He can have it back in 10 minutes if he hands it without complaint. If he complains a little it will be 15 minutes, if a lot, half an hour. You can set a timer!?
Overall, I would prefer the former strategy as this is not phone-specific but a general lesson about doing as you have asked without aggression, and for a more 'addictive' activity like a phone it will be harder for him to control his emotional state without having had a lot of practice in less stressful situations.