Breton is more likely since he is from high rock, Atmoran not so much since Atmora is a dead frozen wasteland and there hasn't been a boat from there since the first era and even then the boat was filled with tons of dead Atmorans.
He was a Nord.
Being born in High Rock doesn't make him a Breton.
If a Breton was born in Morrowind, is he a Dark Elf?
BS and you know it. His friggin name is Hjalti Early Beard. NORD
No, no. He was a Breton. He was born in High Rock. Everyone knows you're a Breton if you're born in High Rock.
Even if you have scales, a tail, and look, sound, and smell like an Argonian. If you were born in High Rock, you're a Breton.
All mudcrabs in High Rock are Bretons, too.