DC has been playing videogames nonstop and watching countless hours of Youtube since mid-March. Meanwhile, one of his classmates finished Don Quixote and is doing science experiments at a medical lab. Sigh. I know I shouldn't compare him to other kids, but it is SO HARD when it is clear that my kid has been goofing off and slacking while others are being productive. How do I get my DC to actually learn something or read something worthwhile?

Mine is doing the same. Goofing, slacking, gaming. I think the Don Quixote kid is in the minority, but like you, I am despairing of how to fix this, especially without the imposed rigor of in-person school.
I'm in the same boat. I don't know what to do except limiting screen time (but I think my kid is younger than yours, so that might be hard for you to do). Good luck, OP.
OR above and I have a teenager. Limiting screen time is hard by this age and doesn't even feel like a viable long-term solution (they'll have to live with the temptation of screens for the rest of their lives).
I could have written this post! At least when they're in school I can control screen time on phones with time limits. Now they are on computers all day, and it's not like there are a ton of social or sports events happening (nor should they) so it's really hard. I should have done a home pod or something... but we can't continue with the online learning much longer. It's very frustrating and I hear a huge academic and social emotional slide.
I understand you, OP. I have one kid of each type. What I’d like to add is that the one who is less driven is not “worse”. She is incredibly kind and creative. She has her own benefits and is as valuable to our family and will be one day, to society as the other DC. I would encourage you not to compare. It won’t help you or him, I know from experience.
How old is your DC? Rather than focusing on limiting screen time (which I agree is a losing battle by the teen years), I think it's perfectly acceptable to say that, as a family, you intend to be going above and beyond what is required by the school. Ask your DC to outline goals, including a reading list, a legit hobby (i.e., not gaming!) or activity that can be done even with social distancing. I even know some parents who created a reading list from which their kids (high schoolers!) need to select from; or parents are asking them to read the newspaper and discuss current events over dinner. Or they could take up baking/cooking, sewing, drawing, learning about the stock market--whatever grabs their interest.
Wow, you sound like an amazing parent! My kids are seemingly depressed and don't want to engage in anything really. I will try some of this (not OP but similar kid)
@Anonymous I'm the OR. My DD likes to go thrift shopping and then alter the clothes or design something completely different from the fabric. She has really become good at sewing. She also took up drawing and is pretty good. She didn't do much reading this summer--which we were kind of disappointed about. She's a little older (18 and starting college), so it's harder to force something. But we did make it clear that she couldn't sit around idle all summer. When she was younger, when she wanted to drop an EC activity, she had to have a plan for what she would do instead. We made it clear that the time wouldn't go to socializing, You Tube, Grey's Anatomy reruns, or social media. That seemed to work pretty well and she does have a lot of hobbies (and always has).
thank you for bringing this up- so many of us are struggling with this exact issue right now