Having ADHD makes him more susceptible to video game addiction, because he’s always needing/looking for that dopamine hit.
He’s also more likely to find it difficult to organise himself so long ‘to do’ lists as pre-requisites might not be very helpful and could be counterproductive in your relationship with him.
You say he’s a good kid and getting on ok at school. These are HUGE plusses.
In your shoes, I would learn as much as possible about ADHD. I would also, very gradually and in a non-bossy way, start talking to tour son about the life he wants for himself. His future job, relationship, family. His health. Start talking about healthy lifestyles, the benefits of exercise, of meeting people and spending time with family. Help him develop an interest in ADHD too, and learn about the benefits he can lean into and the pitfalls he will need to actively avoid.
The aim is for him to develop a vision of what he wants for himself, and for it to be a healthy and achievable vision. It can include gaming, but he needs more than just that to live a well rounded life.
Google motivational interviewing. There may be some tips there you can use.
You will get posters here telling you to use rewards and punishments and to turn off the wifi at 8pm. All ways to manipulate/force him to go through the motions and live the life you want him to.
This doesn’t work with teenagers, they are hardwired to want to do their own thing and will just resent you for trying to control them.
They key is to get alongside him, find out what he needs and wants out of life, feed him helpful information to help him get there, and approach life together as a team- at his pace.
(p.s. if he says all he wants out of life is to be a videogamer, ask him why? What does it bring him? Is it the adrenaline? The satisfaction of developing a skill? The social kudos? Recognition? The feeling of progress? THESE are the things that matter to him, the game is just one way of finding them. Are there other ways he can find these in addition to gaming? So he can live a well rounded life? Sounds like the sport is one way - there will be more.)