DS1 is just 8yo and a year into his orthodontic treatment.
He has a very narrow and high palate and there would not be room for all his adult teeth if it was left. His bottom jaw was also bigger than the top, he could not put his bottom teeth behind the top ones.
He has braces now (top and bottom) to expand the palate and lower jaw and he wears headgear at night to draw out the middle section of his face which was very flat, restricting his nostrils.
This is a very common orofacial structure in people who are tongue tied. DS1's was never picked up as a baby/toddler, but when he started having gastric trouble aged 5yo, I discovered he was tied (it was posterior, so not obvious, but also a substantial upper lip tie).
A tied tongue cannot make the correct swallowing movements and the palate, which is normally smoothed out by the tongue in utero and in the early months of the baby's life, does not flatten out properly. He was tongue thrusting too, which meant his tongue was going forwards instead of up to the roof of his mouth when he swallowed, which was causing an open bite.
I would suggest that you investigate the possibility of tongue tie. What you describe is very typical in tied people. Orthodontic work which is done without correcting the root cause (the tongue tie) can be less effective or even become "undone" over the years.
Be aware that most HCPs do not understand about tongue tie. Some have taken a specialist interest, but at the moment, they are few and far between.
I was poo pooed by the docs when I said that ds1's many problems were caused by his ties and our NHS dentist shrugged when I mentioned the high palate. I went privately in the end, and revision followed by orthodontics has been great for ds1, he is a different child altogether now he is free of gastric problems, is eating better, sleeping better, breathing better etc. We still have a long way to go with the orthodontics, but I can really see progress being made. Totally worth it IMO.