I'd probably start with the aaracokra stats, so that the wings get handled. I'd also give him claws that function like short swords (light and finesse) and a bite that functions like a longsword. That'd be the "race."
For the class, I'd use the fighter, but tweak it so that the dragon "subclass" came online at 1st level, replacing the fighting style with Breath Weapon.
And I'd give him two breath weapons to start. Maybe three. One breath weapon would be the equivalent of a 1st level spell (and would be useable once pet short rest), the others would be balanced like cantrips, like a firebolt equivalent would be spitting on a single foe, while an acid splash equivalent would be a small cone. Then, at higher levels when the fight would get an archetype feature, his breath weapons would power up and he'd get more.
And if I came up with more dragon features I wanted to hand out, I'd replace other fighter features as desired.
I'd also write this up as a whole new race/class combo without reference to the aaracokra and fighter so that it would feel dragony.