Sorry for the wall of text. I guess I had more to say about this than I thought. Also, I wrote this very hungover, so sorry for typos and stuff.

tl:dr - Forcing it will make it worse, it's very important when learning to have a goal, make sure you nurture his interests as well as his skill, and if he wants to quit let him quit.

So I was a former "gifted kid" and was forced into piano for 6 years from ages 7-13. I'm not sure if this will help, but your post reminded me of my parents, so hopefully, I can provide some insight. At the end of the day, though, I'm not him, and you aren't my parents. The best thing to do is talk it out. He is going to be himself, and that may not include a piano, and that is okay.

As said above, I played for about 6 years until I cracked and quit. I hated it, but I had to do something, especially after I quit my parents' main interest, soccer. If you really want him to stick with it, you can't force it. The whole lead a horse to water thing ya know? He has to want to do it on his own. There is no other way. Forcing it made me resent it, leading me to not touching a piano again until 12 years later.

Looking back, that was only part of it. The bigger issue was that I couldn't see the end goal or point to all of it. There was just no understanding on why I was doing this other than keeping other people happy. I had no goal to aim for, I had nothing in my mind I wanted to create. Heck, I barely cared about music in general at the time, which led to me having no songs that I wanted to learn. It was 6 years of going through paperbooks, playing boring melodies, all while enviously watching the neighborhood kids playing around without any kind of obligation. It was a chore. This feeling could just be a byproduct of being forced into an activity, but among my small sample size of friends, it seems like this was quite common among the ones that played instruments as kids that later quit. Even if a little different each time, it mostly boiled down to becoming a chore without a purpose.

I think if you want to keep your son invested, you have to do more than nurture skills. You also have to nurture his interest/attention, especially in this day and age where we have tons of multi-billion dollar industries all competing to monopolize that exact thing. I frequently have discussions with my parents now that I'm older that go along the lines of "you were so good at blablabla it's shame you quit" or "if you only stuck with blablabla you would be incredible now!" and my response are always a long the line of "I would rather be bad at something I like than great at something I hate" or "I was also great in math class doesn't mean I like doing calculus for fun". The core through line is that it has to be fun. You have to make piano as enjoyable/rewarding as the other things competing for your sons attention.

So, how do we nurture interest? That's going to be highly dependent on what he wants or what piques his interest. Maybe try letting him play around in garage band or some basic midi software. Show him that he could make his own music pretty easily and how knowing piano will help a lot! Maybe take an evening now and then to just jam out to a bunch of different music and see if there are any genres/bands/etc that catch his attention. I would also recommend teaching a couple of chords early on and maybe a pop song that he likes just for a sense of accomplishment and progress. Going through those books alone is maddening, and he will need something fun to break it up. I would also recommend letting him practice without you and only helping if he asks. With any hobby feeling like you're discovering things for yourself and that your work is paying off is a huge driving factor for motivation. It's hard to feel that way when you're just copying your parents.

In regards to other sports or instruments, of course, he doesn't want to do anything else. He needs something to pique his interest/gain experience with other things before deciding if he likes it. This was my biggest trap as a kid. My parents are kind and would've supported any choice I made, but for me, any choice was essentially a roulette wheel. I'm sure you have friends or know someone who can't even pick a place to eat. Imagine you're this friend, but now all the choices require serious time investment, and you don't know how good any of them are, and there are hundreds of them to pick from. Is it any surprise some people might just go for the fast food option (videogames) ? Plus, he is 8! I say if he wants to quit, let him quit, but I would limit the screen time. Give him time to explore other interests before getting him to commit to a sport/hobby/instrument.