When my oldest was having trouble with this at about that age, I pulled a pie out of the fridge and began cutting it up.

"This is one pie cut into two pieces. This is half of the pie. You write this as 1/2.

This is one pie cut into four pieces. This is one fourth or one quarter of the pie. You write this as 1/4. Notice how the pieces are smaller than when it was 1/2.

This is one pie cut into eight pieces. These are eighths. You write this as 1/8. Notice how the pieces keep getting smaller as the number on the bottom gets bigger."

He never had a problem conceptualizing it again. It made complete and perfect sense why a larger number on the bottom meant it was a smaller amount.