Would like to know what your farrier says, and why he hasn't suggested anthing, MAYBE YOU NEED A SECOND OPINION.
If your hose is in a clay, muddy area this will shorten the life span of the shoe staying on, as the mud or clay when dry will swell, therefor pushing the shoe away from the hoof.
As far as riding without shoes, I would not suggest it, as you seem to ride alot, and going bare foot will not harden them, they could make them widen and split, also if the surface you are galloping on is hard you could get splints or worse fracture his peddle bone.
You may also need to look at his diet, is he getting the appropriate vitamins and minerals. Good hay or grass is one thing, but may not be sufficient enough.