It all depends on how many Frank Eliots are still alive in your family by the time your son has a son of his own.
If your father (Frank Eliot, Jr.) and your oldest brother (Frank Eliot III) are both still alive by the time your son (Frank Eliot II) has a son of his own, then your grandson would be correctly named: "Frank Eliot IV".
However, once your father dies, your oldest brother would thereafter correctly be known as just "Frank Eliot", your son will still be "Frank Eliot, II", and your son's son would be "Frank Eliot III".
Should both your father and oldest brother die, your son will then become just "Frank Eliot" and his son would be "Frank Eliot, Jr.".
Sorry, but unless your family is royalty, you don't get to keep your Roman numerals indefinitely.