I knew pretty much all my classmates last names when I was in elementary school, middle school, etc. Teachers used last names, there are yearbooks, many families lived in the neighborhood so we knew their names, and so on. I would've noticed if a 1st grade, 4th grade (or any grade) classmate all of a sudden changed their last name. So thinking nobody is going to notice a name change is highly unrealistic to me. Hopefully the transition will go well for your son.