If it's only "...and a sandwich" why must we put 'aige' which also means he has
Because, unlike English, Irish doesn't have a verb for "to have". Instead, they put ag + pronoun at the end of the things he has. So if he has rice, it'd just be Tá rís aige. But, since he has rice and a sandwich, you need Tá rís agus ceapaire aige.