Poor lad, it is tough when you have your heart set on something and it falls through and when you are young, hard to see the light at the end of the tunnel. You sound like a really supportive mum.
Let him talk, don’t try to placate him or help him to look on the bright side with “at least…” type statements as these will only make him feel unheard/misunderstood.
When he feels ready you can look at alternatives together.
He has dodged a bullet. In service he will experience high stress and trauma as a matter of course and need to keep calm as a matter of life and death. Certain personalities are definitely more suited to doing this. Having a record of anxiety, no matter if it was in the past or not, will be an indication that a person is more than likely not the right fit for the pressure of the job.
For example, my husband (ex career military) went out for a run and found a lad who had been stabbed, horrific but he saved the boys life while others who were there panicked. He didn’t know if the attacker was still there or not. He came home covered in blood but perfectly calm, spoke about it once then that was it. I was waiting for it to hit him for him to need to talk about it, it never did. There were other people there (some who knew and cared for the lad) who in the moment were incapacitated by fear and anxiety, their fight/flight kicked in, it wasn’t in their control. No matter how much they wanted to save their friend they were unable in the moment. Personally I found seeing my husband in that state, and the smell of the blood, traumatic and that was after the event.
Hope he finds something he loves doing that he is suited to op. Good luck to him.