My observation is that you are both very young, maybe this is simply a relationship you’ve grown out of as you’ve matured?
You say you’re concerned leaving would destroy him…. But if you stayed against your better judgment that might also be harmful to both your happiness? I don’t think I’d be happy if I realised my partner was dissatisfied with me.
He sounds a great guy, and he’s young, do you think it’s possible he might in time find someone a better fit for him?