"essen" is irregular in the present tense and the vowel changes in the second and third person (du/er/sie/es). As with gender in nouns, you just have to learn it as there isn't a rule for why a verb is regular or irregular.
ich esse
du isst
er/sie/es isst
wir essen
ihr esst
sie/Sie essen
Sandwich is a neuter noun in German. While the masculine article is "ein" in nominative and "einen" in accusative, the neuter article is "ein" in both nominative and accusative. Thus, "Er isst ein Hähnchensandwich".