We adopted a baby boy some years ago, who was also converted then. As I grew up abroad according to Nusach Ashkenaz, and pray now in a Shul in Israel where Nusach Sepharad is predominant, but Ashkenaz is also used, and I pray according to Nusach Ashkenaz, which Nusach should I teach my son, when he is growing up? I personally feel comfortable in both, yet feel my son would be better off davening in Nusach Sepharad, as it is more widely used, but if so, then should I do so as well? Any recommendations?

Halachacly a son should follow his fathers Nusach, if you use Ashkenaz that is what he should use as well. It can also confuse a child to use a Nusach different to his father’s.