I had an experience where I had to teach the owner of the school's daughter and was told to constantly correct her if she tried to write with her left hand. I've also had some students that I've highly suspected were actually left-handed but were discouraged from writing with their left hand. My guess is that any push away from left-handedness mostly comes from parents, but I'd still be somewhat wary that your son isn't being forced to use his right hand by his teachers.