Purely technical limits often don't work when the incentive behind it is too strong - he needs a real reason to put the phone down, not just a block. Maybe try concrete agreements instead of bans: like phone-free times during meals or before bed, and in return he gets peace and quiet the rest of the time. At the same time, it's worth having a genuine conversation about the worse grades - not accusatory, more like "hey, I've noticed that..." - because then maybe he'll see for himself that the phone is hurting him. Also important: lead by example yourself, otherwise it comes across as hypocritical 😅