One high school teacher's take, and not an exhaustive answer.
Most kids get into learning most when they can see that it's important to, and possible for, other people who matter to them: their peers, their family (in teacher's college I read articles on how parent involvement in a school is a good predictor of student motivation), their teachers, and the society they hope to be a part of someday. If you're lucky, you have a kid who needs little enticement and enjoys knowledge for knowledge's sake, but for most students, that enjoyment only comes — if at all — after they've already waded in over their waist for social reasons and start to like the water.
Is the teacher engaged and passionate? Are they able to show it in class? Can you email or call them and ask what you can do to make their job easier? (Maybe you can pick up the supplies for that lesson tomorrow that they just don't have time to get.) Can you ask them for their perspective on how your son is in class and any insights they can share? Can your kid get them to talk about their favourite aspect of their subject? Can you coax a story out of them? Can you get them to show exemplars of their favourite past projects and talk about what makes them so great? If you're particularly social and brave, can you invite the teacher over for a meal?
Is your family a part of the school? Do you go to school events? Do you attend the parent-teacher interviews? Do you show an interest in your kid's teachers and classmates? Do you make connections between what he's learning and what you learned? Do you invite his siblings to share their memories of the class? Do you discuss your son's work, e.g. reading a report at the dinner table or looking at a piece of art or talking about research or listening to him practice his instrument or reading a chapter of his English book or drilling his French vocab?
Does he have friends who enjoy school? Do his conversations with friends turn to what was good in class (could you encourage this as a mental health practice: list one good thing that happened in class each day)? Does he have study get-togethers or only video game get-togethers? Does he ask his friends how they're finding the homework and bring up any questions that confused him? Does he offer to help them understand the topic? Have they compared their lists of things they love when teachers do and things they hate when teachers do? Does he choose to vary who he works with in group projects? Has he ever joined a club or band or a sports team with one or more friends, just to hang out with them, even if he wasn't automatically interested in that club? Has he ever stayed after school with a friend who needs homework help, to keep them company, or asked the same of one of his friends?
Does he know what the purpose of his learning is? Has he looked at university or college programs that need the classes he's in? Has he looked up which math is used in which trades and careers? Has he looked up the pathway to becoming a video game developer, zoologist, athlete, journalist, parademic, diplomat, or what have you? Has he watched interviews with his heroes where they talk about their childhood? Has he read the book or graphic novel versions of his favourite TV shows or movies? Has he watched interviews with his favourite athletes speaking three different languages? Has he watched documentaries about issues connected with his courses (climate, history, justice, military...)? Have you taken him to a museum or art gallery with an excellent exhibit connected to something he's learned — remembering that there are lots of museums about non-historical topics? Has he listened to podcasts and looked up the things they talked about, e.g. a true crime podcast and read about the laws or forensics that made it an interesting case?
Just a bunch of ideas and directions to start with for connecting school to his life. The beauty is that on this side of lockdowns, these things are possible.