Should I let him sleep?

Let all old people sleep when they can . Some have more trouble falling asleep . Let them be on their own schedule that works for them . My parents took naps every afternoon when they were old. They went to bed very early as well . They slept a long time overnight . When their 60th wedding anniversary was coming my hubby asked my Dad what they were going to do for their anniversary. My Dad said, “ take a nap”.

“Never wake a sleeping husband or sleeping baby.” Rose Fitzgerald Kennedy

As lealonnie1, cwillie and AlvaDeer have written, your husband is now firmly in the world of Alzheimer's.
I am dealing with almost the same thing here-my husband has Frontal Temporal Dementia and over the past few months is sleeping until 1pm. He wakes up, has a cup of coffee, fruit or yogurt, watches some tv and is now playing a computer game. He has set this routine, and it seems to suit him. I'll check in on him, see if he wants a snack or something to drink. Dinner is at 7pm, we watch a movie and then he's up to bed by 9pm or so. Because he was thrashing in bed we now sleep in seperate rooms. His room is set up with the computer, new mattress for the bed, and soft lighting. It's quite nice. He may go to sleep around 1am. I do not know, I let him have his time and schedule as he sees fit. I do remind him every evening, that he must wake me up if he needs anything. Otherwise, this is the routine right now. I know it will change as his brain is affected by dementia. At least now, he seems somewhat content.
The neurologist is aware of the increase in sleeping and thinks it's fine, as does his primary care doc. No need to insist on re-joining a world dementia will not let him be part of. It's not his fault, it's the disease taking him away from me.

Even without dementia added into the mix, the elderly tend to want to sleep more and more and more. My Dad had no dementia, but he sure did love to sleep and nap, and toward the end of his life admitted to me that, though he had had a wonderful life, he was just longing for bed and sleep.
Only you know Paul's nature and his diagnosis, and his norm for his every day. But it is perfectly normal to want to sleep a lot. You might want to consider sleep patterns for the night not to be very disturbed and wakeful.
There is no right and no wrong, basically, when dementia is there. It is as unique to each person as his or her own fingerprint.

Leave Paul alone to do as he sees fit with regard to his sleep habits, that's my suggestion. Expecting an Elder with Alzheimer's to strictly adhere to someone else's idea of "healthy sleep habits" is absurd, imo. Your goal is to enable your husband to live his best and happiest life now, that's all. Throw some of society's rules out the window because his disease doesn't play by those rules. Nor does it respond to logic or reason. It's our job to enter THEIR reality now to further enable their comfort and prevent agitation. If Paul's neurologist doesn't understand that basic premise, you may want to find him a new doctor. It's shocking the number who do NOT understand a thing about the disease or how to handle the patient!

AD will rob your husband of everything he is or ever was, bit by bit. Strive to keep him comfortable until you can't do it anymore and then look into Memory Care Assisted Living when things get too tough.

Take care of yourself along the way too. Your life is just as important as Paul's.

Best of luck.
Best of luck.

His neurologist is either trying to humour you in your complaints of his sleep habits or they are not very well versed in dementia, because screwed up sleep schedules are incredibly common and very difficult to treat. If I were you I'd call with a reminder for breakfast at a reasonable time simply because he likely has medications he should take at regular times and it encourages more normal bathroom habits, then allow him to go ahead and nap again until noon... but I also wouldn't fight over it if it's not possible.