I know it's very irritating, but you do have to just keep repeating - and as far as possible try not to sound irritated or impatient. He simply can't remember what you said before - that is the sad fact of the matter - he's not being deliberately difficult.
You do have my sympathy, though - my FiL would ask the same questions endlessly - I once counted 35 times in one hour! I could not show the slightest irritation, since he was apt to fly into furious rages. I made a sort of separate compartment in my head, where I could go on answering pleasantly, but mechanically, if you see what I mean. And I am not the most patient person in the world, far from it.
I know it can be hard to comprehend what's needed, though. There was an occasion where my sister and BiL were visiting my mother, before she was bad enough to need a care home. My sister was in the kitchen, cooking, and about every minute and a half my mother would call out, 'Anything I can do?' To which my sister would reply, 'No, it's fine, thanks.'
But eventually my Bil became very irritated and snapped, 'Stop hassling her!'
My mother was very upset, because of course she had no idea that she'd been saying it over and over.
I had to explain to BiL - who'd never lived with dementia - that she honestly couldn't help it, her short term memory was shot to,pieces, so it was no earthly use getting cross with her.