Mental health services are horeendously overworked. There are still psychogeriatricians (for 65 years plus). He will need a GP referral, though- or a diagnosis from GP. Dementia is very common after a stroke- however, because most medications are for the symptoms and not the cause - a diagnosis might not change his medications or treatment anyway.
There are support places, such as MIND or Care Support who can give advice and support to people in your position. From his behaviour, i had actually assumed he had dementia anyway- i hadn't realised it wasn't confirmed. It must be very difficult to tell the cause though, as lack of short-term memory could be from dementia, or from other damage to the brain, such as a stroke. I don't know if you'll get a definite answer. Good luck, though xx