I don't think the specifics matter that much, what you are looking for is dividing lines that can create conflict.
In your example, say you decide the son doesn't want to work with his father. That alone creates a conflict, a potential rift between father and son.
As for the rest of your questions, I would avoid "what if" questions per se, you need to focus in on something more specific.
I would ask, "What does the son want to do instead that will widen this rift between father and son?"
Then don't write down answers or questions about all these professions. You can consider and dismiss them. I presume you have already chosen a profession for the father that allows a father-son team. So you can try to think of a profession for the son the father would consider a waste of time and money, or would despise, or would consider too dangerous, or would simply not be proud of telling his friends about, should his son adopt that profession.
For example, if the father is a proud cop, he might be fine with his son becoming a soldier or lawyer, but not fine with his son pursuing acting.
But if the father is an actor, vice versa: The actor might think becoming a cop is too dangerous, and too risky for mental health. The father believes the job, being exposed to real violence, anger and resentment on a daily basis can cause radical shifts in the psyche, and he doesn't want to lose his son or see this radical shift toward calousness in him. The son wants to do it because he thinks being a cop is truly helping real people all day, and acting is self-indulgent and playing pretend, he thinks his father never grew up and just stumbled into making money by playing. He doesn't want the free pass his father can give him into the business. Because life is about other people, and the son cannot imagine more pride in himself than actually saving the life of another person, to him that would be real and concrete. For him, he thinks you don't save lives by dressing up as a wizard and pretending to cast spells in front of a camera. He's grown up behind the cameras, watching his father, and the only person he's seen there with an important job is the medic on set!
The same goes for other brainstorming; you don't need to get too detailed, because at this point you are mapping the broad landscape. Like me above, once you find something that lets your imagination take off, you've found a fault line. Move on and find another. If I'm focused on the son, where is the big bad fault line in becoming a cop? Corruption? Killing somebody? Accidentally killing an innocent person?
You have to grow a sense of "looking too hard". For me, I know the right idea when I get it, a kind of Eureka moment. So I think about the questions for awhile, if I feel like I can't come up with something after awhile, I'll put it aside and come back to later.
If I get stuck on the story altogether that way; it means I've road-blocked myself with some earlier choice, and need to change it (Eureka moment or not). They say in writing you must be prepared to kill your darlings (passages you love that don't work), and this is a variant of that.
Remember you are looking for big fault lines, the Grand Canyon divides in your story, the impassable rivers and mountain ranges. We don't care about nitpicky details, because these divides are about emotional conflicts for the characters. Decide the son wants to be an "athlete", that's all the detail you need, not whether he wants to be an Olympic swimmer or a pro tennis player. The little details are for story time, and whatever is convenient or easy for you to research; because the little details will only have little or no impact on the emotional landscape. They can be entertaining, but the big emotions are about big life decisions, and those are about life's turning points (as you intuit in your question -- Does the son want to work in the family business?).
Stick to the big picture, there are only about a dozen decisions to make in a story (look at the Three Act Structure), and you only need three or four to get started writing (which will help you in the remainder of the story). The opening is about the MC's normal world and life without any big conflict. You need an inciting incident, that grows into a crisis, that forces the MC out of their normal world. After you have written that quarter of the book (Act I), you will have better luck brainstorming the other three quarters, each with three or four decisions to make.