I am guessing my experience was not the norm... but it could give you some helpful insight.
I wore braces for just over 10 years. My teeth were in very bad shape, however, the very extended period of time that they were on my teeth ended up doing more damage than good.
10 years is just too long to wear braces for.
The biggest problem was that I was started way too early with braces. I hadn't even lost my baby teeth yet so they had to keep pulling them out at various points of the treatment.
Had we waited until I was several years older (I was started in braces at the age of 9) my orthodontic treatments would have lasted only a few years (4 maximum) and I wouldn't have ended treatment with terrible tooth decay from wearing the braces for too long.
I think that orthodontic treatments are excellent when you have a great orthodontist and they are started at the correct timing.
I can't give a suggestion on age other than 9 being way too young. In my opinion you should have all of your adult teeth (except wisdom teeth) before you get braces... and you should also find out the full plan of the orthodontist as well as their record before you begin treatment.
I went through 3 orthodontists. The first one began while I was too young because they were greedy for money, the second one was incredible and tried to repair the mistakes of the first but he sadly died of cancer and I ended treatment with a third orthodontist, thankfully he was a good one too. Both the second and third orthodontists agreed that I was started too early in braces.
My orthodontic needs were pretty intense, have extra teeth on top and bottom, severe overbite and severe overcrowding. knowing this we realized that I'd need a longer plan than most and more treatment than most, however, 4 years would have been more reasonable than 10 in my situation.
It is also good to have an idea of the outcome. I still have an overbite, though thankfully it is nothing nearly as bad as what I had prior to wearing braces and going through many treatments. I've now had my braces off for almost 10 years and my bottom teeth have started to go a little crooked in the front. I'll have to have them moved slightly ~ this is common when braces go on in adolescence for some teeth to move later on. It can happen when wisdom teeth come in or for a variety of other reasons.
Because my braces were left on for 10 years I have some teeth that had severe decay. I have needed numerous fillings, root canals and even a few implants. All of which could have been avoided if I had worn braces for less time.
Only the orthodontist can give you specific answers on your son's situation and what his needs are. If you are in doubt of the treatment they plan to give him, wait and get another opinion.
Also a lot of orthodontists deal with children who have difficulty going through dental treatment. They usually have neat things to help them have less anxiety during their visits.
Wishing you the best!