There are pros and cons to attending the accepted-student events, so your son will lose out in some ways but gain in others by not being able to take part.
One advantage of these events is that they would allow your son to eyeball a big batch of his potential classmates. Granted, it will be impossible for him to tell which ones will eventually enroll, so perhaps the gaggle of Goths (or lax bros) that most (or least) attracts him could be gone by September. These all-comers events are also usually well organized and rife with panels and other formal presentations that allow prospective students to get a helpful glimpse at a broad swath of the college experience (academics, athletics, clubs, etc.) in a short amount of time.