You have not mentioned his age
He could be having actual dementia
- brought to prominence / made obvious by the stress
Surgery and anaesthesia may have contributed to the unmasking.
Intensive care set up contributes. Withdrawal of sensory inputs in hospital settings precipitates this in many people.
Encourage / feed the senses - // Talk to him; turn on music or news (hearing) // Touch and comfort // allow to read or see TV // and so on... Thing may improve - though there is no guarantee
Tender loving care - turn many around