There are many reasons why people yawn. Being tired is the most common. Lack of attention, being bored, being bombarded with too much information are also reasons why people yawn. The common thread in these are that the person is no longer interested in receiving information.
For example: Two boys start learning B’Chavrusa and suddenly one is constantly yawning. Its not so much that the Gemara is a “sleeping pill” its just that this boy isnt of the mind to sit and learn at the moment.
If your date is yawning every 2 minutes you can talk until youre blue in the face he will not remember a word you say once the date is over. I do not think its rude to say,
“I see you are tired, you mustve had a long day, I think we should cut this date short.”
You will come across as caring and it will prevent hard feelings on all sides.