"Mangio manzo, mangia maiale" was my guess, but duolingo wants me to use "lui mangia". I disagree using only 'lui' is necessary, its either 'io' and 'him' to stress who's eating what, or none at all. Does this make any sense?
"Il carne di maiale" is wrong: carne is feminine, so "la carne". That being said, with this little context both are acceptable; you can say maiale to refer to both the animal and its meat.
Probably because "i" is thought to be a mistyped "di" in the sentence. Carne (d)i maiale. However, stating the wrong gender of a noun will not be accepted.
Porco is never used to refer to the meat (at least that I'm aware of); it isn't even all that common to refer to the animal, because it's considered somewhat impolite.
What's wrong with maiale = pork? :P A pork cutlet is simply "cotoletta di maiale". Porcine would be "suino", it's common on labels but pretty rare in the spoken language; be careful not to confuse it with the similar "porcino", which is a variety of fungi (also "penny bun" or "cep" in English).