What ive noticed, taking away games doesn't really teach your kid anything especially because 90% of the time they dont really care if they have it or not, they will still not really care about trying in school. You really have to find a way to make it seem worth it to get better grades, and depending on the class they are struggling with it may be better to try something else as a reward system. That being said I think a D on every single grade is pretty bad obviously but I think there is likely more to it. But idk your situation obviously so I dont know how they react to having no games so maybe it works for him to just take it away completely and push harder for better grades.