You don't do the 'dont forget to...' reminders/nags.
You need to get him to act there and then.
Teenagers will think of a dozen reasons not to do something, and have a dozen ways of starting a fight instead up their sleeves.
Just tell them to do it, do it now and keep repeating yourself. Don't explain, don't justify, don't debate, stay calm and just repeat.
If needs be folow them round the house turning off laptops, wifi, xbox etc and keep saying 'do it now'.
Once you hit the amusing heights of maximum annoyance they will do it. Nearly always.
Homework I'm about to try something I read, and that's to fix the homework time, fix the duration and just make your teen sit through it. In a public place in the house, not their bedroom.
Even if they claim they have no homework, just to get them to sit. They can play music but no social media its just a case of getting them to sit still for a minute outside of their cave. Key success factor is not engaging with them apparently so they have to find a use for their time.
Theory is eventually they will just use the hour to do homework rather than protest.
I'll double check I read that correctly and let you know how I get on.....