What can be happening to him? The doctors can't find anything wrong.

Was any medication started several weeks or a few months before this started happening? Has he had any neuroimanging (CT, MRI) or other workup done at all? (Not sure what is meant by "can't find anything wrong" - could mean anything from a normal physical without a decent neuro or mental status exam, versus a massive workup with no positive results.)

I don't have any experience with short term memory loss in the elderly and I'm not a doctor, but there's a site called HealthTap where you should go to ask these questions and get it answered for free by doctors. It's always a good idea to get a second medical opinion on these things. Here's a question someone else asked about short term memory loss in the elderly:

Try it and let me know if it works for you. Good luck!

Fever and diarrhea indicate he had a serious infection or parasites. Doctors rarely check for parasites in people, but always suspect them in pets. Go figure.

I'm not sure how he was tested. Perhaps his medication needs to be reviewed, some medications affect people in different ways.