Hello.
I am sorry for the situation your boyfriend is facing. Whatever the diagnosis was earlier, it is obvious that he is depressed at this stage. This may be called post-psychotic depression.
I was wondering if he has been prescribed medicines and is taking them. He may have to add anti-depressants to his treatment regime. Depression is known to complicate a number of mental health problems. Schizophrenia is one such condition.
If he is open to therapy, you should try to seek help of a psychotherapist for him. He needs to replace his core "evaluative" beliefs. You can help him see the situation from a different point of view. Scoring a D does not mean he is not adequate. You could help him understand this.
His self-esteem is very low probably. You could tell him that he should learn to love himself for what he is .... and not what he does or doesn't achieve. Once he realizes that scoring a D is not "that terrible" or awful, he will start thinking more rationally.
I hope this helps.
Regards
This post is for the purpose of providing medical information and is not a substitute for a face-to-face consultation with a doctor. This post is not intended to give or rule out a diagnosis, create a doctor-patient relationship or replace an existing one.