First off, I agree with others who are recommending the direct route. I'd suggest that tactic first, unless you have reason to believe he may get pissy because he's the boss's son. (Again, as others have said, you're probably in a better position to judge that than we are.)
I try to answer in a non-commital but polite way, however he talks and talks and talks.
If I understand this statement correctly, you're saying that you answer the work-related question quickly (but politely) and then try to disengage, but he just moves on to less relevant topics.
Instead of being non-committal, you might try committing to the question more completely. Ask for surrounding details. Maybe walk around to look at his monitor so you can see what he's asking about. Make sure you answer the question fully, including any follow-up or incidental concerns it may raise.
Then, ask him about his progress in general. Something like:
"So how's it going on this project? Is there anything else I can help you with?"
That sort of thing. Prompt him for more work-related information to keep the conversation from going personal. I understand you may not be in a position of authority over him, so try to make this conversational - like you're curious - not like you're demanding progress updates or anything.
If he tries to change the subject abruptly to something irrelevant, you can politely steer him back to the work issue:
"That's cool, Bob, but let's stay focused on this question of yours. I want to make sure I've answered everything you need for the task at hand."
When you reach the point where he no longer has work-related information to ask about:
You: "Anything else I can help you with?"
Him: "No, not really"
You: "Great, I'll let you get back to it then"
And then immediately get back to your own work. This way, you're not just becoming non-committal in the conversation, you're finishing it, while still being cordial and polite, and fulfilling your duty to help him get up to speed.
After a few rounds of this (hopefully) he'll get the hint that you want to stay focused on work.