I think it would be good to set a clear time in the evening when the phone is in another room, like my mother used to do with the TV. Here on the bus, I see teenagers every day who can't stop chatting and their notes look exactly like this. Have you tried giving him a fixed time for homework and only after that the messages?
My daughter and I also had this happen last year - after school we immediately chatted with a friend until the night. I introduced a rule: the phone is on charge in the kitchen from 19:00 until homework is done. It helped, my grades improved. And Ion, all the teenagers on your bus really are on their phones, like zombies.
They both say it well. My wife and I put our daughter's cell phone in the kitchen after 8 pm since last year, just like Tamara says. It stays there charged until the lessons are over. In the morning on the bus I also see the same zombies that Ion says, and I think that if you don't set a limit of small ones, then it becomes difficult. What time did you set your limit?
My son and I also had this happen in high school—after school, I immediately started texting and forgot about my homework. I started putting his phone on the kitchen table from 19:00 until he finished, as Tamara wrote. It helped, his grades leveled out, and now he’s already used to it. And on the bus, yes, those same zombies with phones, it’s scary to watch.
My daughter and I also had this happen last year - after school, she immediately goes to the chat room and seems to forget about her lessons. They started putting the phone on the kitchen table at 19:00, as Tamara writes, until she does everything. It helped, the grades really evened out, and now she doesn't argue herself anymore. But those "zombies" on the bus that Ion mentions are already sad, for sure.