No, I've not been in this situation. My PWD is my mother, and although she was a keen traveler pre-dementia, she stopped wanting to travel very early on in her illness. I have seen reports here on TP from members in a similar situation to you, though, so give them time to respond.
I wonder if you would consider a different approach in responding when your OH asks about holidays, to see if you can avert an argument and upset?
Perhaps agreeing that you will call the travel agent "on Monday" or "next week" or when the agent "gets back from his holiday" or similar?
A sort of "yes, and" approach, that I've seen talked about here on TP. Of course you aren't actually going to book the trip, or buy the car, or sell the house, or whatever the argument is about, but you deflect the discussion so as to avoid triggering an argument or distress, since that doesn't help either of you.
Depending on his short term memory, you can just lie and say you have booked a trip and then be vague about the date, or that you're waiting for confirmation or something. Of course that trip was overbooked, or cancelled, or rescheduled, or something.
It is a variation on love lies and he compassionate communication ideas, if you're familiar with those?
I'd also wonder if he might have anosognosia, as well as possible denial, and it might be counterproductive to try to reason with him. Its pretty common with dementia and basically means, because of the damage to his brain, he can't understand or comprehend he has a problem, so to him, of course there isn't anything wrong. It can be tricky to deal with.
I hope you're able to find something that works, so you can avoid all this stress and upset. Best wishes.