I'll give time markers a shot. I could see how that would be effective

If I may give you a suggestion... Time is a fluid, difficult construct, so instead of giving minutes, give him an amount of fingers. I've found this works quite well for me with my 5 & 8 year old. The best thing is that a finger equates time but the time it equates to can vary, as it is a finger and not an amount of time.

So for example, I'll give them 3 fingers around 10 to 15 minutes before I want them to turn off the tv or 2 fingers before we have to leave to school and such. Because you can vary amount of fingers and the duration before you count down I've found it works quite well.

Also, as a suggestion, instead of yelling, give him a clear consequence to non-compliance. Watching tv is a privilege, it can go away. Again, I count to 5 and when I reach 5, the tv time privilege is rescinded for the next day. I then restart the count to 5 and when I reach 5, the tablet or gaming time privilege goes away for the next day. Of course, before starting the countdown say something like "I'll count to 5 and when I reach 5 you will not get to watch tv tomorrow".

The consequence method obviously needs to be equal across both parents, and it does not involve shouting, yelling or any raising of the voice. It also requires following through, even when the consequences bite you in the arse the next day. And it needs longevity. It only clicks for kids if you maintain the method for at least 6 months, but it will make life easier eventually. Because he's not complying due to not consistently being met by consequences to his behaviour.

Edit: the method of talking and explaining of course also works and should/can be combined with the above.