Actually it seems like the subject the OP chose would be a good thing to ask Dad, with sincerity, once things have calmed down ("What reaction would you like?"). What I see are people coming at the same conversation with different agendas and expectations, and they're irritated when those clash.
Dad did some work on behalf of his son--he talked to the personal chef for quite a while about her job, asking her questions that he didn't have to ask. If he wants to share that information, and be thanked for it, he needs to communicate it better, and not get offended when people can't read his mind after a single vague opening line. Making people pry information out of him to show their interest, even when the interest is completely feigned, is not a polite thing to do, it's just playing power games.
On the other hand, if DS assumes Dad is gearing up to do another dull, off-topic monologue, he needs to find a better way to deal with it, by redirecting the conversation, listening politely, asking on-topic questions, whatever. Some experimentation/discussion may be required. I think his longer response about not wanting to be a personal chef pretty clearly indicates that he thought whatever his dad was about to say was not going to be relevant to him, and I'm guessing from Dad's response that this came through in the "okay."
Other people have suggested more articulately why this might have been a poor assumption from a knowledge/connections point of view. Also, assuming someone is going to say something boring and irrelevant before they say more than one line seems dodgy to me. You may be completely correct in your assumption, but I don't think you ought to let them know you've assumed that, you know? It's like in Emma when she tells the dull girl, "Hey, you're dull, and we don't want to hear your dull stories anymore." (paraphrased) Everyone knows she's dull, but everyone else politely pretends otherwise, and extricates themselves without hurting her feelings, because they know she means well.
But regarding "means well"... I think it can be a fine line between showing interest but not getting it quite right due to lack of knowledge, and willfully not getting it quite right because you want to push the person down a slightly different path. The former can probably be looked upon with affection in some cases, while the latter may seem self-centered and controlling. (For another Jane Austen comparison, picture Lady de Bourgh using her social power to make everyone sit in her parlor all day and listen to her dull stories, which often include criticisms of her guests' behavior.) A lot depends on context and personalities. If you generally get the sense someone disapproves of your life choice, you may feel their off-target comments are subtle criticisms, and you may be right or you may just be over-sensitive. On the other hand, if you generally feel someone is supportive of your life choice, you may feel their off-target comments are their attempts to connect with you despite their lack of knowledge, and appreciate them.