I was a teen with braces and undiagnosed SPD. A “prickly pear”. (I also needed glasses around the same time, so I was a real cutie!)
I’d say wait as long as possible, in case of a growth spurt. This will give him time to learn tools to manage discomfort and frustrations.
My family was/is pretty dysfunctional but I’ll share what I wished for/know now:
Buy those fuzzy-tipped flossers and small toothbrushes. String floss won’t be a good option. Make oral care as easy and fun as possible.
Make eating as tolerable as possible for the most painful times after adjustments, which I think lasted about 2 or 3 days…it was a long time ago! Unfortunately, mac-n-cheese and nut butters are orthodontic disasters. Soup. [Thinly-sliced candy bar (for a treat that can melt on the tongue- like a 3 musketeer).] Jello. Smoothies if using a straw doesn’t hurt. Peeled grapes. (I did that! Sometimes I still do- I have food quirks.) Peas. Yogurt. Little stick pretzels.
Hugs. Or at least just being there. And a long-sleeved shirt for chilly offices.