score:7
This is, presumably unintentionally, a trick question.
There were no Dukes of England, Great Britain or United Kingdom who were elevated in the Victorian era in their late twenties to early forties, on the death of their father, while unmarried and while their mother was still alive!
Even if there were, this question is, effectively, unanswerable. In terms of precedence, the new Duke (not heir the moment their father died) would be higher than his mother. She, as you say, would not be styled dowager until the Duke married. She would remain the Duchess.
How they were served would depend to some extent on when in the Victorian era. Early on, etiquette would have been to bring all the food for the course out at once and the diners would serve themselves. Even for formal eating. Later on a la russe service became popular and, with it, being served.
For a private meal, either could be possible as could a buffet, especially for breakfast or tea.
One final point, during the Victorian era, it is highly unlikely that such a meal, even an intimate one as described, would only have one server present. We are talking a level of nobility one down from the monarchy. Even the "poorer" ones would have had a significant household. And some of them were amongst the richest people in the world at the time.