When he was at high school, he developed / was developing a passion for computer animation.
When he was at high school, he developed / was developing a passion for computer animation.
developed là hành động hoàn tất trong quá khứ – quá khứ đơn.
Dịch: Khi còn học trung học, anh ấy đã phát triển niềm đam mê với hoạt hình máy tính.