See if you can get an appt. with a therapist or couples counselling, maybe if he hears this from a person outside of your relationship, it may sink in.
Would he be willing to? If he wants to keep the relationship he needs to know how important this is.
If he is living in his created world, then it pretty much seems what you mention maybe part of this problem, avoiding reality, maybe he is in denial of having a DX.
Denial is common when one has been handed a DX, but in time it can be accepted and live with it, medications can help tremendously with some talk therapy too.
The therapist may help draw out of him what may be causing him to withdraw from the RL.
I wish you lots of luck, it is a difficult situation if or when a patient doesn't want help, but if compliant things can go well.

BTW
Is he in therapy already? Is he prescribed any meds?
Do you think all of this is more of a maturity problem?
I think starting off talking to a therapist/counsellar may help you both.

Take care now,
DE