Suppose this is England in the Victorian Era and there are two individuals, a duchess and her son, dining together. The son, roughly thirty or so years of age, is seated across the table from his mother. His father, the duke, has recently died and left the title to him, which makes him not only heir apparent but literal duke, I presume. As mentioned, the son is dining at the manor with his mother, the duchess (but not dowager duchess, since the son is currently unmarried--I believe that is how it is decided, but please correct me if I am wrong!). If there is only one servant serving food to the two of them, who would this servant serve first? Would it be customary to first serve the duchess, because of her years and bearing, or her son, who has recently acquired the title of duke? Does the latter circumstance matter? What if the duke was still alive but absent from this meal--would the duchess or the heir (being male) be served first? Thank you in advance!
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.