We discussed something similar in "Can you ask questions that involve lists where you want to learn the items on the list but also what is best?"
In my answer there, I quote an early blog post by Jeff Atwood on the topic of what can make a subjective question a good (constructive) question: Real Questions Have Answers
Constructive subjective questions:
- inspire answers that explain “why” and “how”.
- tend to have long, not short, answers.
- have a constructive, fair, and impartial tone.
- invite sharing experiences over opinions.
- insist that opinion be backed up with facts and references.
- are more than just mindless social fun.
At a glace, I also considered the question you link to to be 'primarily opinion-based'. However, the OP did a great job at defining it narrowly enough so that it wouldn't elicit an infinite list of responses and asks explicitly for resources which could help his son himself making up his mind. He is not asking for answers to tell the OP what to choice (quite meta, but good meta).
As your answer on Quora indicates, you can answer this in a fairly non-subjective way which lives up to the criteria listed above.
My vote is to keep it (and 'subjective' questions with similar characteristics) open.