What countries has your son been to? Usually with regards to travel history, they want to see someone who has travelled to countries requiring a visa, so that they can see if the applicant has a history of getting visas and obeying the immigration laws in those countries. I've seen lots of people here who have traveled to many countries not requiring a visa, but still be refused under travel history.
Anyway, I don't think you should think too much about this refusal. This is more common than you realize, that spouses and children would be refused. A person could have a strong application but refused, or a weak one but approved. Part of it is due to luck as well. Instead of wondering how the hell CIC could refuse a 12 year old boy a TRV, just focus on what you can do to improve your 2nd attempt. If it's refused a 2nd time, I would honestly just forget about bringing him here on a TRV and just wait for the spnosorship to go through, since each failed TRV attempt just makes it harder to be approved the next time.
He has been to three countries but only Vietnam required a visa. One has to start somewhere. When we talk about the length of stay and consistently I see that shorter is better but the logic of that in this case is confusing. I know it relates to ties to home country but if the intent was to overstay two weeks is no different than two months.
I wish the vo was the one who had to explain to a young man why others can get in to Canada but not him. Very difficult to present that rationally.