I was friends with a guy in high school named
Alexander nn
Sasha, and although I didn't understand it at the time, I am rather fond of in now, as is he (he hated it back then). My advice would be to use
Alexander as a fn and
Sasha as the nn, because as a child he probably will not care for it, but as he gets older and his peers get older he will grow to appreciate the historical nature of both the fn and the nn. (hopefully)