As far as I know, topical fluoride treatment only strengthens tooth enamel. This makes it harder for acids that cause cavities to stick to the teeth.
How is ds-with-cavities at brushing/flossing? Does he have the patience to brush twice a day for at least two minutes each time? That might be the reason for the difference... different brushing habits between the boys.
(As a kid I *hated* brushing my teeth, so while Mom was getting my sister ready for school or bed, I would run my toothbrush under the water and then say I had brushed my teeth. LOTS of cavities! I came to my senses late in high school, and I've had two small cavities in the seven or eight years since. Oh, and I had regular fluoride treatments as a kid. Brushing makes a MUCH bigger difference than fluoride does.)
How is ds-with-cavities at brushing/flossing? Does he have the patience to brush twice a day for at least two minutes each time? That might be the reason for the difference... different brushing habits between the boys.

(As a kid I *hated* brushing my teeth, so while Mom was getting my sister ready for school or bed, I would run my toothbrush under the water and then say I had brushed my teeth. LOTS of cavities! I came to my senses late in high school, and I've had two small cavities in the seven or eight years since. Oh, and I had regular fluoride treatments as a kid. Brushing makes a MUCH bigger difference than fluoride does.)