Is he a reader? Could you check out a bunch of books and/or audio books and make that part of your daily routine?
Would your kid be amenable to cooking or baking? You could involve him in making one meal per day. That can easily fill an hour and will also teach life skills.
We have so many great museums in the area, and you could easily spend 2-3 hours at one each day.
Seems like a good time to organize wherever your child does independent play—his bedroom, the living room, a playroom. Put toys, crafts, games, and books into organized bins on shelves. Sit down and create an “I’m bored” menu with your child. When he complains, keep redirecting to that menu. Be consistent. He’s old enough to manage independent play, and it’s a battle worth fighting. Make yourself busy with a book, a workout, or meal prep and refuse to walk away from that, no matter how many times you’re interrupted.