My son has been taking it for about 4 years now.  When he first started taking it a doctor told me that the first boys who had been taking it had gone through puberty and they had viable sperm.   My son hasn’t had any noticeable symptoms from it.  It works by blocking estrogen.  Estrogen is what causes bone age to advance.  Theoretically, pre-pubertal boys do not need estrogen.  If your son’s bone age is advanced already, I might see if you could use Lupron to stop the puberty in addition to or instead the letrozole.  Or do they not want to stop puberty because he’s ten?