Hi Stezzle,
If his HyperT meds need adjusting you will find that out in May so no problem if its associated with this condition.
Sadly feline dementia is quite common especially now cats are living much longer. there is not much you can do, there are some supplements avavilable but I have no experience on how effective they are - ask your vet when your there next.
The best thing is reassurance though, which i think you are doing already 
If he is still enjoying playing then enjoy yours and his games. Maybe just let him out when you are in the garden with him so he can turn to you if he is wondering what's going on xx
Ive been eating my lunch in the back garden and letting him outside with me. He seems ok with that. He will on occasion jump over the back fence which is up to him. I wont force him to do anything. He is of course sleeping most of the day and goes outside for about an hour at 7am, then an hours at 1pm and then its around 9pm til 11pm that he is in and out. Then he is inside after 11pm because he has his medication then.
His eye sight is perfect (playing with toys) and his hearing is good too.
Just have to see what the vet says, again, it could just be his way of aging and i am getting all worried about it but i wanna be prepared and offer him the best quality of life. I made him a promise and i intend to keep to my word. (we promised that he wouldnt run away and die, and i would protect, love, and care for him until that day comes, no matter what. I would also do the right thing by him when it came to the day that he just couldnt keep fighting it)
Thanks guys,