Learn Spanish (Latin America): A él le duele el estómago porque come mucho.

The verb doler (to hurt/ache) works just like gustar. Instead of saying "He hurts," Spanish phrases it as "The stomach causes pain to him." Le means "to him," and the verb duele is singular because it matches the thing that is doing the hurting: el estómago.

Why do we need A él at the beginning if we already have le?

Because le can mean "to him," "to her," or "to you (formal)." Adding A él at the start clears up any confusion by specifying exactly who is feeling the pain. If we already knew who we were talking about, we could just say Le duele el estómago.

The English translation says "his stomach", so why does the Spanish say el estómago (the stomach) instead of su estómago?

In Spanish, you almost always use the definite articles (el, la, los, las) for body parts rather than possessive words like su (his/her). Because the sentence already uses le (to him), it is already perfectly clear whose stomach is hurting!

Why is it spelled duele? Doesn't the verb come from doler?