I'm sorry he is in this situation.
As everyone will tell you here, each injury is different so what happened with one case may not apply to another. I will say that my DS also received a LOA (for USNA) in early fall but then was DQ after sustaining an injury which required surgery.
Has USMMA requested a waiver for him? You must ascertain that first to be able to tell where the delay may be coming from. He will be able to see if USMMA has requested a waiver when he logs into the Dodmerb site. If they have not, considering he has an LOA, it would be worth contacting his admissions officer. My DS had the LOA and was already qualified through Dodmerb when his injury occured. He informed Dodmerb of the injury and went from Q to DQ in his portal. A waiver request from USNA appeared in his Dodmerb portal about 2 weeks after he was DQ.
He didn't wait for an AMI request to come, but instead provided Dodmerb with what he deemed would be the required documentation showing he was fully recovered and fit for service. He could tell what those docs were by looking up his single disqualification code somehow on the Dodmerb site and he requested those docs from his doctor's office. He was 18 by this time so there was a form he had to sign at the office to be able to get those released to him.
For him, along with documenting he had fully recovered, he needed a certain amount of time post surgery. Once that time arrived, he proactively submitted everything to his Dodmerb case manager, who then replied within a day that docs were received and would be sent to the waiver authority (USNA) for review. It was about a week before med changed from DQ to Q in the portal and then the appointment came shortly after that.
If he has gathered the docs needed to show he is fit for service he could submit those to Dodmerb. If he hasn't done that yet, have him start requesting those ASAP as doctor's offices need time to gather everything.
Good luck.