Earlier quoted context omitted.
Disclaimer: not a parent. He's a preschooler, not a teenager or even a kid. Can't you just... not give him a phone/tablet except at specified reward times?
I am a parent. You absolutely can. The kid will make your life miserable about it for anywhere from 4 hours to 3 months depending on how stubborn they are.
We still watch stuff together every now and then (or if we really need to be focused on smth), but he's in a much better mood throughout.