I was waiting for your reply.
I am writing the answer as I noticed you are offline.
It is possible to recover from delirium if there is any metabolic disturbance.
Low sodium or hyponatremia, low glucose etc. can cause confusion, delirium or semi comatose state.
Infection like pneumonia is also a frequent cause for confusion in elderly.
If those are corrected then he can recover from delirium.
Blood levels should the checked regularly.
As the doctors are probably thinking that he will recover from this state so they are proceeding for invasive procedures.
However, if he has not got an evaluation by neurologist, he can have one.
Stroke, TIA need to be ruled out.
I hope this information helps.
I will be happy to assist, if you need any clarification.
Please feel free for any follow up query.