If this is the case, his feet are contracting, squashing the frog. Have you had this horse a long time? Has he had this farrier a long time? I'm wondering if his feet have changed noticibly to you or the farrier? Recently?
Treating the frog itself will not help, as the changes need to be made to the whole foot, the whole horse. This can be due to a number of problems, such as high heels, shoes, insufficient hoofcare, toe first landings, insufficient exercise on firm ground, etc. The horse needs a *balanced* regular trim & possibly a change in management practices to start the recovery of his feet. Every hoof is different however & while some are, or can become broad, open & flat accross the heel & frog with proper care, some horses will always have slightly 'contracted' looking feet & slightly higher heels. In domestic horses, this is largely governed by the type of care they had as youngsters, while the foot was still developing. Therefore hooves should not be forced to decontract using invasive trimming. Take a look at www.barefoothorse.com
for some more info.