My kid just quit the swim team, which she was excited about joining for years. Turns out, she doesn't enjoy swimming competitively as much as she does just swimming for pleasure. I was a little bummed, because it felt like she was giving up and I think she could be awesome at it if she stuck with it. But in the end, if she wasn't enjoying it, there was no compelling reason for her to be doing it.
I think this is normal. Kids try stuff. It's hard and they quit. They get scared and they quit. It wasn't what they thought it was going to be and they quit.
They don't have the experience going into it to know that this thing might be hard, or this thing might be different than how you imagine. All they know is "Oh..... this sucks." It's going to particularly suck if they see other people around them doing the thing. You'd think that would motivate them... Nope. it kind of just drives home the fact that they can't do the thing. So they get frustrated.
It's human nature to want to avoid things that frustrate, frighten, and make us uncomfortable. Perseverance and determination are traits that have to be learned and it's going to take a while to sink in. It's not like a toddler learning "don't touch the stove!" The kid touches the stove once and gets burned, therefore they have no desire to touch the stove again. Hearing "Keep trying, you'll make it!" when you've just witnessed yourself fail 20-30 times in a row.... that's harder to stay motivated against.
Pick your battles. What stuff is worth encouraging him to stick with and keep trying. Algebra? Yeah, probably. Squat thrusts? Not so much. Find the stuff worth sticking and and keep encouraging him. Stay positive, and help him develop better strategies for accomplishing his goal.
Knowing want to keep working at and what to walk away from is an important lesson for him, as well. "Never give up" is admirable, but sometimes it's just more stressful than it's worth and making a conscious decision not to let something stress you out isn't giving up. It's choosing not to participate.