I am a Professor of Piano myself, I started piano in China,

learned "Do, Re, Mi" which is called Solfége, it's an older teaching system. Most students learning Solfége have perfect pitch. In America, teachers usually teach "A,B,C,D..." because kids are familiar with the alphabet, so it's easier to learn to read music. Personally, I think if you understand Solfége, it's not hard to learn the letter system, your son will eventually need another teacher but for now, it might be a good chance for him to learn Solfége. Hope this helps.