We discussed this very question today in my Hawaiian class here in Hawaii. My teacher said that pono, means morally correct, proper. The opposite of pono, or" pono 'ole" means unjust, dishonest, unfair, wrong. People don't answer "pono" to the question "How are you?" They answer "Maika'i". Hope this helps.
The meaning is quite different. He is doing well - a personal statement about status - health, wealth, relationship, whatever.
He is doing good - a political statement about the ethical quality of the life being lived. Huge difference.
Different words, different parts of speech, quite different meanings.
Ia is the pronoun "he/she/they/it", ʻO is an untranslated word that has various functions that I don't fully understand yet, but in sentences like this one it is typically said to function as a subject marker: since it comes before ia, it marks ia as the subject. ʻO doesn't usually come before pronouns other than ia, but it often comes before proper nouns (Maikaʻi ʻo Mary, Mary is good), and before the interrogative wai (ʻO wai ia?, Who are they?)
In regards to gender, ia is not inflected in such a way. It can mean he, she, they, or it. It typically serves as a filler pronoun for an aforementioned noun, so whatever it refers to is inferred by context. In a contextless sentence like the ones Duolingo provides, you can translate it however you want.
The audio sounds like the letter A in the word Maika'i is pronounced as in the month of May, and then the A in in last word (ia) is pronounced like the A in Father or About. I thought that Maika'i would be pronounced like the vowels in the expression "My kite". So, if the woman speaker in this recording is saying it correctly, then how would you pronounce MEIKE'I ?