I’m not sure HS was ever fun so I personally would not make that my goal. Instead, I would talk over goals with the kid. As I said, I have always thought that the goal was to help the kid have a happy, productive, fulfilling adult life. I then try to work backwards with them over proximate goals and how best to meet them. But the OP and his son can think about the goals they think are most appropriate.
A friend of mine is a foreign-born CEO of a pretty big company. He asked me to talk with his very bright son who was only interested in math/computers and didn’t understand why he should try to do well in some of the other courses. Despite remarkable success, my friend and his wife don’t have any credibility with the son because neither had experience with the US educational system. So, I spent a couple of hours with him working backwards from longer-term goals and explained the game of getting into a college. We talked about the benefits that come from playing the sorting game and how to minimize the costs. I left the choices up to him. His father tells me that the son is now applying himself very diligently following the strategy I suggested and the father is very pleased.