If the condition of your DS's LOA is the dental issue, it's a DoDMERB thing, not so much an Academy thing (other than if the braces aren't going to be removed prior to I-Day, there needs to be a waiver).
What I mean by that is that you can breathe a little easier, in that the "April 15th deadline" is only when they propose to get out most Offers of Appointment.
Offers will often be delayed by medical paperwork and waivers - so if all the information that DoDMERB has requested is already in to them - you just wait.
Now, if your DS sent in the remedial paperwork (you wrote "waiver", but I think you meant the answer to the DoDMERB remedial that will be reviewed for a waiver by the Academy) in March, it wouldn't be considered bad form to send an EMail to the Admissions Counselor to make sure that they have the paperwork and that they aren't waiting on anything from him.
Remember that the LOA means that they want him to receive an appointment.
If the paperwork complies with what they require, a waiver will be granted and an Offer will be tendered.