The Shulchan Aruch (OC 174:2) rules:
יין פוטר כל מיני משקין: הגה אפילו מברכה ראשונה (הר"י פרק כיצד מברכין וב"י בשם הפוסקים):
Wine exempts all types of drinks. Rema: Even from a first Beracha.
The Mishnah Berurah (§3) comments:
כל מיני משקים - ובלבד שיהיו לפניו על השלחן בשעה שבירך על היין [דאלו לא היו לפניו בשעת ברכה אף שהביאו המשקין לפניו בשעה שעמד עדיין היין לפניו לא מהני וצריך לברך עליהם] ויש מקילין אפילו לא היו לפניו ורק שהיה דעתו עליהם
All types of drinks - Provided that they’re on the table before him when he made the blessing on the wine. For if they were not before him at the time of the blessing, even when they bring the drinks before him at the time that he stands, and the wine is still before him, it doesn’t help, and he needs to make a blessing on them. Some are lenient, even if they were not before him, just that his intent was on them.
וכ"ז בשלא קבע עצמו לשתות יין ורק כוס זה בלבד אבל אם קבע לשתות יין הסכימו כמה אחרונים דפוטר אפילו המשקין שלא היו לפניו בשעת ברכה כיון שבאו לפניו עכ"פ בעוד שהיין לפניו [ואם באו לפניו אחר גמר שתיית היין צריך לברך על המשקין כ"כ מהר"מ בנעט בביאורו אם לא שהיה דעתו על המשקין קודם גמר השתיה] ולכתחלה טוב יותר לעולם שיהיו שאר המשקין לפניו בעת ברכתו על היין.
All this is when he did not establish himself to drink wine, just this cup alone. But if he established to drink wine, many Achronim agree that he exempts even the drinks which were not before him at the time of the blessing, since they come before him in any event when the wine is still before him. If they came before him after he finished drinking the wine, he needs to make a blessing on the drinks, if his intent wasn’t on the drinks before he finished drinking. Ideally, it’s always much better that the other drinks be before him when he makes the blessing on the wine.
ודע שאם קידש על היין והוציא אחרים בברכתו אם לא טעמו מכוס של קידוש ורוצים לשתות שארי משקין אף שהיו לפניהם בשעה שבירך על היין צריכים לברך על המשקין כי הטעם שיין פוטר הוא משום שכל המשקים טפלים לו וכשאינו שותה לא שייך טעם זה ועיין בבה"ל מה שכתבנו עוד בענין זה:
And know that if he made Kiddush on the wine and fulfilled others[‘ obligations] with his blessing, if they did not taste from the cup of Kiddush and want to drink other drinks, even if they were before them at the time that he made the blessing on the wine, they need to make a blessing on the drinks. The reason that wine exempts is because all drinks are secondary to it, but when one doesn’t drink, this reasoning doesn’t apply.
Based on the above, I see several explanations for the argument between your Rabbi and father-in-law:
- Your Rabbi is Sefardi, while the father-in-law is Ashkenazi. Only the Rema says that a first-Beracha is exempted; the Shulchan Aruch doesn’t agree to that.
- Both are Ashkenazi, but the father-in-law interpreted that he always goes to drink scotch afterwards as meaning that your Rabbi had the scotch in mind when he made Kiddush. Your Rabbi didn’t have the scotch in mind, and so it wasn’t included.
- Your Rabbi held like the first opinion cited in the Mishnah Berurah, that the drinks must be before him when Kiddush is made for them to be exempted, while his father-in-law held like the second opinion, that even if they’re not before him, as long as he has them in mind, they’re exempted.
- The father-in-law interpreted that he always goes to drink scotch afterwards as meaning that he was establishing himself to drink, so it would be included. Your Rabbi held that it’s not a drinking session; perhaps this is because it was interrupted by the Mezonos, perhaps this is because the later drinks need to be wine to fix it as a drinking session.