My father and his family went to nearby grandparents for some Jewish Holidays and sometimes Shabbat meals. He has a strong memory for foods, flavors, sweetness etc. Once when I gave him a baked apple, he said:
"My mother used to make baked apples. Hers were larger and much sweeter."Yes, I admit that I don't add sugar.
Considering my father's state, he may not quite get the explanation of Shavuot as the Holiday celebrating G-d's giving us the Torah. The Torah never meant all that much to him. Being a Jew was something he has always been proud of, but religious observant is less important. When my father was in the United States Navy during World War Two, he was among the few Jews to admit to his religion. That's why he was called "The Jew."
The chances are that he'll remember that there was a holiday in the spring/summer when they ate cheesecake and blintzes. Maybe that will bring back some memories.