I am so sorry to hear about Dex!
Is Dex an indoor/outdoor cat?
Are you worried that he may have got into something toxic or something that might be upsetting to his stomach?
There are number of reasons why a cat or dog will stop eating and drinking. Obviously some things can develop suddenly and other condition can be progressive. Sometimes pets will have some clinical signs days to weeks leading up to an event and then all of a sudden they just feel so bad that they can't hide it anymore.
Have you noticed any changes with Dex in the past few weeks?
Any increases in drinking or urinating?
I think the first thing to do is to determine if there was anything in the environment that could be causing him stress or anxiety- a reason for not eating or drinking. If it is really hot, that can be a factor. I would recommend trying to put him in a cool calm place. If nothing has changed in the external environment, then we have to think about things that could be affecting him systemically. In older cats we have to consider not just digestive issues, but also other organ problems such as kidney disease, liver disease, thyroid disease etc. Obviously in order to diagnose an internal problem, a blood test would need to be completed by your veterinarian.
Since this seems to be the first day that his is going on....If he seems relatively stable and alert, then I would suggest keeping him in a cool quite place. If he doesn't have any history of digestive issues, then you can always try giving him some low sodium chicken broth, chicken gerber baby food or just plain chicken breast to see if that triggers an appetite.
If the problem is not resolving, then you may have to take him in to be examined to ensure that there is not a more serious problem.