How do I tell him he's changed? We're not friends but I care about him due to crush...?

I think you said it best in your PPS. So, if you can get him alone in a low key setting, gather your courage and say, "Lukas, there's been something on my mind and I just want to get it out because you've been kind to me in the past and made me realize things about myself and I want to do the same for you because I care about you and about what others think of you" Then the rest of what you wrote.