Be gentle, but consistent and always outline how routine makes his/her life better. For instance, lay out your desk a certain way and you'll finish your work earlier and have more time to spend gaming. Acknowledging productive tasks that allow for dealing with pent up energy is also important.

My parents for some reason thought hanging negatives over my head always mentioning "think of what would go wrong if you don't do the right thing". Honestly, this just left me with a bunch of hugely negative self-talk issues that took a while to unpack since many of those things also had to do with my ADHD.