Problem Hooves - Help... I want to ride!

GRANDMASIS asks: "I have a large, heavy horse with big, flat feet. His previous owner had him shod because of the rocky terrain in the area. He is now in a flat, soft dirt and we want him to go barefoot. The problem is his hoof wall chips away and leaves him walking on his sole. Our farrier said there is not enough hoof to put shoes on. He is not lame but very tenderfooted on hard surfaces. What can I do to toughen the sole and outer wall. Additives to diet? Pine tar? Easy boots? Help....I want to ride!"