He is very observant during Ramadan and strictly follows the fasting rules.

observant 是形容词，由动词 observe（观察、遵守）加上后缀 -ant 构成，表示"观察敏锐的"或"严格遵守（宗教规定）的"。在宗教语境中常指"虔诚遵守教规的"。